Relax test skip conditions for the following colorspace
ImageTests -

- SourceRenderbufferTargetTexture_Colorspace
- SourceRenderbufferTargetTextureExternal_Colorspace
- SourceRenderbufferTargetTextureExternalESSL3_Colorspace

For these tests, it is sufficient to check for support of
EGL_EXT_image_gl_colorspace extension

Bug: angleproject:3756
Tests: angle_end2end_tests --gtest_filter=ImageTest.SourceRenderbufferTargetTexture*_Colorspace*
Change-Id: I123fb14fc79e8f949a903b7586a0badf54297cfa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2144549
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>