Build: Bump tf-m-tests

To include psa_can_do_cipher related tests.

Signed-off-by: Meggyesi, Janos <janos.meggyesi@arm.com>
Change-Id: I7250bf6c750a5266a3eaf3589e167a9b1ac8fc12

Author: janos-meggyesi-arm

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=07d13ea
+version=668ce15d