(cherry picked from commit 7bb8c500)

use ASSERT_FLOAT_EQ to compare float values, it is more robust for
large values
(cherry picked from commit d00f36ec)

use ASSERT_FLOAT_EQ to compare float values, it is more robust for
large values

disable some gpu tests if library was built without CUFFT(cherry picked from commit b4b929d27cc25822dd15b5b96b8d335c59d4408c)

Also cv::, cv::gpu:: and cv::ocl:: namespace prefixes can be safely omitted
inside CV_ENUM and CV_FLAGS

minor refactoring of gpu tests

added more tests for gpu cvtColor
fixed RGB->YUV conversion

added additional tests for gpu filters
fixed gpu features2D tests

fixed some bugs in gpu module

fixed several warnings

wrote more complicated tests for them
implemented own version of warpAffine and warpPerspective for different border interpolation types
refactored some gpu tests

updated gpu tests for CornerHarris and CornerMinEigen
moved direct convolution implementation to gpu::filter2D, gpu::convolve now use only DFT-based algorithm (Bug #1639)