Instead of requiring a special environment variable to skip tests
which require a special X11 environment, skip these tests by default
and have users opt-in with the DO_X11_TESTS environment variable.
Signed-off-by: Brian Nguyen <brnguyen@nvidia.com>
testx11glvndproto tests specifically the GLVendor X extension, which is
used to lookup XID -> screen and screen -> vendor mappings in the
prototype.
testglxnscreens tests that the API library successfully dispatches GL
calls to the right vendor based on this lookup.