test: add scratch to available backends

[tlambert03]

just more test coverage for the scratch backend

[codspeed-hq]

Merging this PR will degrade performance by 50.5%

❌ 1 (👁 1) regressed benchmark
✅ 18 untouched benchmarks
🆕 4 new benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
👁	test_bench_append[t5_c2_z3_xy256-tensorstore]	51.3 ms	103.5 ms	-50.5%
🆕	test_bench_append[large_2d-scratch]	N/A	36.5 ms	N/A
🆕	test_bench_append[3d_chunking-scratch]	N/A	133.1 ms	N/A
🆕	test_bench_append[t5_c2_z3_xy256-scratch]	N/A	21.3 ms	N/A
🆕	test_bench_append[time_lapse_small_2d-scratch]	N/A	36.1 ms	N/A

---

_{Comparing tlambert03:test-all-backend (f7c4984) with main (1ac01ed)}

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.60%. Comparing base (5cc4747) to head (f7c4984).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ome_writers/_backends/_scratch.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
+ Coverage   97.46%   97.60%   +0.14%     
==========================================
  Files          21       21              
  Lines        2285     2298      +13     
==========================================
+ Hits         2227     2243      +16     
+ Misses         58       55       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.