Check that image*Shadow are not reserved keywords

Check that image*Shadow are not reserved keywords

There used to be a tests that treated: image1DShadow, image2DShadow,
image1DArrayShadow, image2DArrayShadow as reserved keywords but they
were removed because the spec doesn't actually reserve them. Any
drivers that were passing the old test were behaving incorrectly so
this change adds new tests that check if these keywords are not
reserved.

Components: OpenGL ES

VK-GL-CTS issue: 3007

Affects:
dEQP-GLES3.functional.shaders.keywords.allowed_keywords.*

Change-Id: Ic6b4463e51b45611441481d661d71735570dd15a