Imgproc test cleanup - #29740
co-authored by: @Prasadayus
### Re-enabled as-is (stale disable reasons)
- `FillPolyFully.fillpoly_fully` (`test_drawing.cpp:1142`)
- `Resize_Bitexact` (`test_resize_bitexact.cpp:188`, 4 instantiations): `INTER_NEAREST` and
`INTER_NEAREST_EXACT` agree exactly at integer upscale factors; measured 0.0 diff on all 4.
### Add assertions
- **imgproc** — new `Imgproc_Watershed.regression`: `cv::watershed` had no working coverage at all.
### Pull Request Readiness Checklist
See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request
- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake
Varun Jaiswal
Alexander Alekhin
Alexander Alekhin
Vladislav Sovrasov