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/include/mbedtls/threading.h b/include/mbedtls/threading.h
index d50d04e..b4df0e3 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -30,7 +30,7 @@
     pthread_mutex_t MBEDTLS_PRIVATE(mutex);
 
     /* WARNING - state should only be accessed when holding the mutex lock in
-     * tests/src/threading_helpers.c, otherwise corruption can occur.
+     * framework/tests/src/threading_helpers.c, otherwise corruption can occur.
      * state will be 0 after a failed init or a free, and nonzero after a
      * successful init. This field is for testing only and thus not considered
      * part of the public API of Mbed TLS and may change without notice.*/