r.watershed: Add comprehensive regression tests

[sumitchintanwar]

This PR adds a comprehensive suite of regression tests for the r.watershed module, which addresses the changes requested in PR #6953.

The new tests verify accumulation, basin delineation, stream networks, slope metrics, and related outputs to ensure algorithm correctness and regression stability.

All tests were validated using the North Carolina dataset (nc_spm_full_v2beta1)

A few older tests were removed as they became redundant after introducing stronger validation-based tests.

Results:
Before: 6 tests
After: 16 tests, All pass with zero errors.

Addresses requests in #6953.

[wenzeslaus]

The tests need to go far beyond testing existing of rasters. You need to compute the data and then test against them in the test. Check some other tests we have for other tools. Also, make sure your new tests don't overlap with existing tests. You may also expand existing test if appropriate.

[sumitchintanwar]

The tests need to go far beyond testing existing of rasters. You need to compute the data and then test against them in the test. Check some other tests we have for other tools. Also, make sure your new tests don't overlap with existing tests. You may also expand existing test if appropriate.

Got it, I understand. That was my bad, should’ve thought about it earlier. I’m working on the changes and will push them soon.

[sumitchintanwar]

@wenzeslaus Thanks for the helpful feedback.!!!
I’ve updated the testsuite to compute outputs and validate them against computed data instead of just checking raster existence.
@wenzeslaus @petrasovaa
A quick review would be greatly appreciated whenever you have time. Let me know if anything else should be adjusted.