Added/Fixed testcases for SIFT

[Yosshi999]

• The testset in perf_feature2d.cpp was a superset of perf_sift.cpp, so I merged it.
• Added an exact regression test for a bit-exact version which will be implemented in the near future. The testdata is: add exact SIFT testdata for bit-exactness opencv_extra#768
• Resolved two TODOs in scale invariance tests.

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 OpenCV (BSD) License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to 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

[alalek]

Thank you for contribution!

Please take a look on notes below.

[terfendail]

Looks like the new test is too strict. Since the implementation at the moment use floating point values and therefore isn't bit-exact by nature there could be small differences depending on compiler optimization options and AVX2 availability.
I think that precision requirement could be relaxed a bit at the moment

[Yosshi999]

The relaxed test already exists. (https://github.com/opencv/opencv/blob/3.4/modules/features2d/test/test_descriptors_regression.cpp#L345)

As we discussed in terfendail#3 (comment) and terfendail#3 (comment), I will withdraw 2d09bfb (with opencv/opencv_extra#768) and re-create a PR when a bit-exact version is implemented.