Add a test for a failure I found with the X Server after my last fixes.
Fix EGL context detection for non-GLES2-core functions.

Fixes failure of the X Server with glamor using desktop GL on EGL, and
the associated testcase in the tree.  This was clearly just
copy-and-paste failure on my part in
b251e3ae337cecda3cbede6fde8cc8f7cef942c9.