Create a module to split test case collection from checks

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/scripts/check_test_cases.py

@@ -18,6 +18,7 @@ import sys
 
 import scripts_path # pylint: disable=unused-import
 from mbedtls_framework import build_tree
+import collect_test_cases
 
 class ScriptOutputError(ValueError):
     """A kind of ValueError that indicates we found