testsuite: Force Mesa to use software GL when asserting about stderr

In an autobuilder environment, there will typically be no hardware GPU available, so Mesa will fall back from hardware to Zink to software rendering. Unfortunately, Zink logs to stderr during loading if no hardware GPUs are available. This particular test asserts that stderr has desired contents, which means Zink's extra output causes the test to fail. We can bypass this by disabling use of Zink.

Resolves: #6478 (closed)