Fix bug in compressed PixelStorageModes tests

Fix bug in compressed PixelStorageModes tests

Compressed pixel storage modes are tested by using the extension
ARB_compressed_texture_pixel_storage.
ARB_compressed_texture_pixel_storage is a core extension since
OpenGL 4.2. Therefore, the cases testing this behaviour need to
check for its support in GL context versions before 4.2.
Without this check drivers that do not provide this extension,
will fail this test instead of being marked as not supported.
This CL adds the mentioned extension checking.

Affects:
KHR-GL33.pixelstoragemodes.compressedteximage2d.*
KHR-GL33.pixelstoragemodes.compressedteximage3d.*

Components: OpenGL

VK-GL-CTS issue: 3020

Change-Id: Ib9a4edeb57995582c07aa11c7db11c6e7bbe1984