This change adds a pw_python_script GN template which defines an action
to run a Python script through a script-runner script. This runner is
responsible for resolving any GN paths to filesystem paths, and finding
output binaries for compiled targets. This allows writing Python scripts
which are ignorant of the GN build system and work only with filesystem
paths.
The unit test runner script is updated to use the new runner template.
Change-Id: I132bb620af2bb1e57e9278fac57b676f8ab5a415
