Add a new test to test_commands.py which:
* Has an incomplete definition of externalrepo
* Defines a local repository, subrepo
* Includes a file from subrepo
* The included file completes the definition of externalrepo
* It also attempts to include a file from externalrepo
This test previously failed with the error message "No such file or
directory: '.../test_commands/tests/test_layers/test.yml'"
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
