Build: Bump tf-m-tests

To include test configuration fixes for the CC312 driver.

Change-Id: Iac1442a737edcfc849a1880428625eb2fbfa1fa9
Signed-off-by: David Vincze <david.vincze@arm.com>

Author: davidvincze

lib/ext/tf-m-tests/version.txt

@@ -9,4 +9,4 @@
 # current TF-M version.
 # TF-M does not rely on this file to build.
 
-version=668ce15d
+version=f4c628d5