walk_compat_sh and walk_opt_sh are basically the same now, so:
* Merge them into one function.
* Use the --list-test-cases option for both of them.
* Rename this merged function as collect_from_script which seems
more appropriate as since it isn't iterating the script but
calling it.
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>