Added a separate script glxenv.sh for the common environment variables used by
the other GLX tests.
Removed the testx11glvndproto and testglxnscreens tests, since both of them
depend on the x11glvnd X extension, which is no longer used.
Removed the unused GLX_dummy_copy stuff from the GLX_dummy makefile. Also
removed the COMMON_* variables, since they're only ever used once each.