Update references to test helpers

Replace:
* tests/src -> framework/tests/src
* tests/include -> framework/tests/include

Except for occurrences of:
* tests/src/test_helpers (since this only contains ssl_helpers.c)
* tests/src/test_certs.h
* tests/include/alt_dummy

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/library/threading.c b/library/threading.c
index 85db243..fde7cea 100644
--- a/library/threading.c
+++ b/library/threading.c
@@ -61,7 +61,7 @@
      * this here in a thread safe manner without a significant performance
      * hit, so state transitions are checked in tests only via the state
      * variable. Please make sure any new mutex that gets added is exercised in
-     * tests; see tests/src/threading_helpers.c for more details. */
+     * tests; see framework/tests/src/threading_helpers.c for more details. */
     (void) pthread_mutex_init(&mutex->mutex, NULL);
 }