Build: Bump tf-m-tests

For the new t_cose patch and fixed NS t_cose test suite
after EdDSA removal

Signed-off-by: G. Varga, Gabor <Gabor.G.Varga@arm.com>
Change-Id: I2ca2a700b5fbc15b4e0ca3ce200ce41f4476d9f5

Author: gaborgvarga

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=8cb7f0d82
+version=6c96d8f51