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>
diff --git a/config/config_default.cmake b/config/config_default.cmake
index 73edfd0..76c1ba4 100644
--- a/config/config_default.cmake
+++ b/config/config_default.cmake
@@ -141,7 +141,7 @@
 set(TFM_MBEDCRYPTO_PLATFORM_EXTRA_CONFIG_PATH "" CACHE PATH "Config to append to standard Mbed Crypto config, used by platforms to cnfigure feature support")
 
 set(TFM_TEST_REPO_PATH                  "DOWNLOAD"  CACHE PATH      "Path to TFM-TEST repo (or DOWNLOAD to fetch automatically")
-set(TFM_TEST_REPO_VERSION               "master"    CACHE PATH      "The version of tf-m-tests to use")
+set(TFM_TEST_REPO_VERSION               "master"    CACHE STRING    "The version of tf-m-tests to use")
 set(CMSIS_5_PATH                        "DOWNLOAD"  CACHE PATH      "Path to CMSIS_5 (or DOWNLOAD to fetch automatically")
 
 set(MCUBOOT_PATH                        "DOWNLOAD"  CACHE PATH      "Path to MCUboot (or DOWNLOAD to fetch automatically")