Add stress tests from "apitrace trim".

Add stress tests from "apitrace trim".

The trim_stress directory makes it simple to stress test "apitrace
trim" by simply dropping new trace files into this directory. The
stress testing ensures that creating a new trace by trimming to a
single frame still generates the same image as that frame from the
original trace. It performs this test exhaustively for each frame of
each trace.

This testing ensures that trimming doesn't break the rendered results
of any frame. It does not do anything to test that trimming actually
discards anything. So the existing cli tests are still
independendently useful for verifying that expected content is
discarded.