twister: Allow deep folder scan

Upon discovery of a sample/testcase.yaml file, twister will disregard
further subfolders. Given the folder structure below, the testcase.yaml
will currently not be discovered:

samples/sample1/
samples/sample1/sample.yaml
samples/sample1/tests/
samples/sample1/tests/testcase.yaml

This is desirable to allow placing test code closer to sample code.

Signed-off-by: Jorgen Kvalvaag <jorgen.kvalvaag@nordicsemi.no>