[Piglit] More tests for 1.40 and TBOs

[Eric Anholt]

Here are the remaining tests I plan to write for TBO API handling.
All that's left I think is to test actual texture fetching (you know,
the whole point of the extension...)

Also included is an updated version of my discard-and-infinite-loop
test.  I think it's a much more interesting test now, though the
trivial version is also included.  Also, it's now part of 1.30
testing, since the spec change was patched back to 1.30 as well.