glx: Test for both strings for EXT_create_context_es{,2}_profile

The spec says, if you export either string, you must export both. This tests add such a check.

The public git history of the opengl registry starts at v4 of this extension, which has the above language. It's not clear to me whether it's legal to export only es and not es2. As ES1-only drivers are vanishingly rare, and ES1-only-hardware running X11 even more so, I extremely don't feel like digging through old khronos svn to find out whether the specs ever had an opinion about this for long enough that code might have shipped reflecting it.