Fix ImageTest.
Fix ImageTest.

This test suite was missing the "GL_" prefix on some of the extension
checks. This was causing every test in the suite to be skipped. Fix
the check and also add some sanity checks that should ensure we expose
certain extensions on particular platforms and renderers.

Was discovered while working on changing the egl::Image class.

Bug: angleproject:2714
Change-Id: Ida89b5c3c33c534e60cbd55c7e1117a6ef6faae9
Reviewed-on: https://chromium-review.googlesource.com/1142963
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle