scripts: Add workflow for "common" directories in find_tests()

Some tests use a "common" directory to store pieces of code which
are reused by different scenarios. In those cases, no test yaml is
found within such director nor within its parents.
If no test yaml is found in a directory, and the directory is called
common, also look in collocated directories if they have test yamls.
If so, add all those locations to the scope.
E.g. tests/bluetooth/controller/common

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>