Build: Set TFM_TEST_REPO_VERSION to type STRING

Fixes issue where it would be treated as a relative path. Also update
GIT_SHALLOW to allow use of commit hashes in the TFM_TEST_REPO_VERSION
variable.

Change-Id: I9c005b795beae9dda3b35fe8c90e370023bb66c9
Signed-off-by: Raef Coles <raef.coles@arm.com>
2 files changed