igt_runner: track the time it takes the test to finish instead of realying on self-reported times in single mode
Currently runner uses the time reported at the very end of a subtest:
Subtest xyz: SUCCESS (20.043s)
This does not account for igt_fixtures executed before and after given test.
In the single mode (default, one subtest = one fork/execv) we can track and use the time the child process lives instead to get more accurate data.