twister: script harness: Also find scripts in subdirectories

When a directory is provided, find scripts in directories and
sub-directories.
This is what users expect.

Note that now we also name the tests not just based on the "basename" of
the file, but on the relative path to the tests.yaml so we ensure we have
unique tests names.

We also sort the globing output.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>