tests/glx: test for a u_threaded multi-context buffer tracking bug

This is a regression test for a bug currently in mesa/main. There is a patch for this bug in mesa!17338 (merged).

In short: gallium/u_threaded doesn't track its bound buffers properly when a buffer is shared between multiple threaded contexts and forgets to update some bindings when a buffer's underlying storage gets replaced during invalidation, causing it to use an outdated or even potentially freed buffer.

Since this fails without mesa!17338 (merged), merging this test should probably be delayed until the mesa fix lands.