Fixes

[ RUN      ] Imgcodecs_gdal.read_fits
ERROR 4: `/build/precommit_linux64/4.x/opencv_extra/testdata/highgui/../cv/gdal/fits_test.fit' not recognized as a supported file format.
/build/precommit_linux64/4.x/opencv/modules/imgcodecs/test/test_gdal.cpp:15: Failure
Value of: img.empty()
  Actual: true
Expected: false
[  FAILED  ] Imgcodecs_gdal.read_fits (0 ms)

.fit format test made optional because GDAL may be built without CFITSIO library support

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• 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
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake