Add a new 'cli' subdirectory for higher-level testing of apitrace CLI.

Add a new 'cli' subdirectory for higher-level testing of apitrace CLI.

As described in the README, this new directory is for tests that
perform higher-level testing of the apitrace command-line interface
than either of the existing traces or apps directories.

Compared to traces, these new tests can do higher-level things like
dumping images from traces, comparing images, etc.

Compared to apps, these tests are intended to be portable across
multiple targets. (This isn't actually the case yet because commands
like "apitrace dump-images" currently work only on OpenGL. Also, even
when that is fixed, we will also need to then extend these tests to
have traces for each supported target for testing.)

In this commit, the only new test exercises "apitrace dump-images" and
"apitrace diff-images" on a simple trace file with one frame,
(tri.trace grabbed from apps/gl).