This change adds functionality to the test runner script to search a
directory tree for generated test metadata files, build up a graph of
test groups, and run a user-specified list of groups with all of their
dependencies.
Tests are run through an external runner executable provided on the
command line. The executable is called with the path to a unit test
binary file.
The old test runner functionality to run a single test directly from its
binary path is maintained, but now uses a runner executable. This is
configured in GN using the build variable "pw_automatic_test_runner",
which replaces the functionality of "pw_unit_test_create_run_targets".
Change-Id: Ic1ed959d87a71266408e1f26e0ae1bf906eebfb0