ci: Pin tf-m-tests version in FIH test

To avoid problems where the latest master of tf-m-tests is incompatible
with the fixed version used by the FIH test. Bump the version of TF-M
used slightly to a commit that allows the version of tf-m-tests to be
easily overridden.

Signed-off-by: Raef Coles <raef.coles@arm.com>