twister: bugfix: Fix infinite loop in test_plan.py script

Commit 72f416f3 added a horizontal
scan for .yaml files when a modification was made in "common" folder.
If yamls were found in such way, the loop ended. However, the
implementation didn't address what happens if such yamls are not found.
This made the script going into an infinite loop. If yamls are not
found next to "common", the script should proceed as before, i.e. go
to the directory above an start looking there.

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