Add CI workflow for tests to match existing README badge

[yarikoptic]

README.md already had a tests badge pointing at
.github/workflows/tests.yml, but that workflow never existed, so the badge rendered as "repo or workflow not found".

Add the missing workflow:

• pytest across Python 3.10-3.13
• coverage uploaded to Codecov from the 3.12 job
• 10 min job timeout (tests fetch remote HDF5 files from DANDI)

ATM ran act (http://github.com/nektos/act) locally ... lapptop is a bit overloaded so it failed with

 =========================== short test summary info ============================
| FAILED tests/test_main.py::test_disk_cache - assert 2.194648504257202 < 2
| ========================= 1 failed, 4 passed in 33.91s =========================
[Tests/test-3]   ❌  Failure - Main Run tests [37.666522389s]
[Tests/test-3] exitcode '1': failure
[Tests/test-3] ⭐ Run Complete job
[Tests/test-3]   ✅  Success - Complete job
[Tests/test-3] 🏁  Job failed
| tests/test_main.py::test_presigned_url PASSED                            [100%]
| 
| =================================== FAILURES ===================================
| _______________________________ test_disk_cache ________________________________
| 
|     def test_disk_cache():
|         tmp_dirname = '/tmp/remfile_test_cache'
|         if os.path.exists(tmp_dirname):
|             assert tmp_dirname.startswith('/tmp/')
|             shutil.rmtree(tmp_dirname)
|         disk_cache = remfile.DiskCache(tmp_dirname)
|     
|         timer = time.time()
|         test_example2(disk_cache=disk_cache)
|         elapsed = time.time() - timer
|         print(f'Elapsed time for uncached read: {elapsed} seconds')
|     
|         # the second time, it should be cached already
|         timer = time.time()
|         test_example2(disk_cache=disk_cache)
|         elapsed = time.time() - timer
|         print(f'Elapsed time for cached read: {elapsed} seconds')
| >       assert elapsed < 2
| E       assert 2.1814591884613037 < 2
| 
| tests/test_main.py:85: AssertionError
| ----------------------------- Captured stdout call -----------------------------
| Loading 1 chunks starting at 0 (0.1024 million bytes)
| Loading 1 chunks starting at 1368288 (0.1024 million bytes)
| Loading 1 chunks starting at 1367167 (0.1024 million bytes)
| Loading 1 chunks starting at 1567739 (0.1024 million bytes)
| Loading 1 chunks starting at 1239144 (0.1024 million bytes)
| Loading 1 chunks starting at 1231694 (0.1024 million bytes)
| Loading 1 chunks starting at 284125 (0.1024 million bytes)
| starting_time: 0.0
| rate: 20000.0
| data shape: (1220698000, 96)
| Loading 1 chunks starting at 650132 (0.1024 million bytes)
| Loading 1 chunks starting at 11596 (0.1024 million bytes)
| Loading 1 chunks starting at 206 (0.1024 million bytes)
| Loading 2 chunks starting at 1 (0.2048 million bytes)
| Loading 4 chunks starting at 3 (0.4096 million bytes)
| Loading 7 chunks starting at 7 (0.7168 million bytes)
| Loading 12 chunks starting at 14 (1.2288 million bytes)
| Fetching 1228800 bytes in 2 threads
| Loading 21 chunks starting at 26 (2.1504 million bytes)
| Fetching 2150400 bytes in 3 threads
| Loading 30 chunks starting at 47 (3.072 million bytes)
| Fetching 3072000 bytes in 3 threads
| Loading 30 chunks starting at 77 (3.072 million bytes)
| Fetching 3072000 bytes in 3 threads
| Cleaning up cache
| Loading 30 chunks starting at 107 (3.072 million bytes)
| Fetching 3072000 bytes in 3 threads
| Loading 30 chunks starting at 137 (3.072 million bytes)
| Fetching 3072000 bytes in 3 threads
| Cleaning up cache
| Loading 30 chunks starting at 167 (3.072 million bytes)
| Fetching 3072000 bytes in 3 threads
| Cleaning up cache
| Loading 9 chunks starting at 197 (0.9216 million bytes)
| Loading 16 chunks starting at 207 (1.6384 million bytes)
| Fetching 1638400 bytes in 3 threads
| Loading 28 chunks starting at 223 (2.8672 million bytes)
| Fetching 2867200 bytes in 3 threads
| Cleaning up cache
| Loading 30 chunks starting at 251 (3.072 million bytes)
| Fetching 3072000 bytes in 3 threads
| Loading 30 chunks starting at 281 (3.072 million bytes)
| Fetching 3072000 bytes in 3 threads
| Cleaning up cache
| Elapsed time: 6.989373207092285 seconds
| 110503440
| Elapsed time for uncached read: 8.661812543869019 seconds
| starting_time: 0.0
| rate: 20000.0
| data shape: (1220698000, 96)
| Cleaning up cache
| Cleaning up cache
| Cleaning up cache
| Cleaning up cache
| Cleaning up cache
| Elapsed time: 1.689589023590088 seconds
| 110503440
| Elapsed time for cached read: 2.1814591884613037 seconds
| ================================ tests coverage ================================
| _______________ coverage: platform linux, python 3.13.12-final-0 _______________
| 
| Coverage XML written to file coverage.xml
| =========================== short test summary info ============================
| FAILED tests/test_main.py::test_disk_cache - assert 2.1814591884613037 < 2
| ========================= 1 failed, 4 passed in 31.31s =========================
[Tests/test-4]   ❌  Failure - Main Run tests [34.873787224s]
[Tests/test-4] exitcode '1': failure
[Tests/test-4] ⭐ Run Complete job
[Tests/test-4]   ✅  Success - Complete job
[Tests/test-4] 🏁  Job failed
Error: Job 'test' failed

but others seems passed nicely before etc... Let's see what CI says