trim_stress: Fail test at first frame which doesn't match reference

When stress-testing large traces, it's inefficient to work through the
entire trace before reporting an error that occurred in one of the
first frames.

Instead, check each frame one at a time immediately as each rendered
frame becomes available. Then fail the test at the first mismatched
frame.
trim_stress: Also test any traces in the directory trim_stress/local

This is useful for performing testing on traces that are too large to
be considered for include in git, or for traces that cannot be shared
due to proprietary contents.
trim_stress: Only look at files matching [0-9]*.png when counting references

If there are any differences from a previous run, there will be
several additional image files left around, (.thumb.png, etc.). And we
don't want to count those.
cli: Add testing for recently-added "apitrace trim --trim-spec"

Each test here is quite simple. Without doing any image-based
verification, simply ensure that the expected calls are trimmed with
each different invocation of --trim-spec.