add basic tests for fence create/poll/export
Fence export tests require VRENDTEST_USE_EGL_GLES
to be set.
Because there is no way of knowing whether fence export works or not, I have to add detect_fence_export_support
. On Mesa 20.2.4, that seems to confuse the driver and, when virglrenderer is initialized again, Mesa crashes inside eglCreateContext
. Mesa ToT is fine.