piglit: Parse and report subtests

We want to handle subtests from piglit tests. This is a key feature for parity with the piglit framework, and somewhat overlaps with #9. Some of the tricky bits in my mind:

• run_caselist_and_flake_detect() needs to be able to return tests not in remaining_tests
• baseline can handle subtests
• flakes can handle subtests
• skips may or may not handle subtests, I don't have a strong feeling
• subtests don't count against the total tests to run for the purpose of estimating remaining time
• Do something reasonable when we get subtest results but not top-level result (some piglit tests have done this).

We're going to want piglit integration testing to make sure this all works.