features: re-enable & triage DISABLED tests - #29752
Requires: https://github.com/opencv/opencv_extra/pull/1404
### PR Changes:
### features test cleanup
### Re-enabled
- `Features2d_Detector_MSER.regression` (`test_detectors_regression.cpp:47`). Two things were needed:
- `MSER_Impl` overrides `detect()` and had dropped the empty-image guard its base class carries, so the
harness's 0x0 probe reached `detectRegions`, which rejects anything under 3x3. Added the guard; FAST and GFTT
already carry the identical block.
- The stored baseline held 215 keypoints against the 210 MSER finds today. The drift comes from
`minDiversity` entering region rejection in `fa73b91e39` and the later `fitEllipse` rework.
### 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
Prasad Ayush Kumar
WU Jia