Access the test data mutex via accessor

Remove the use of extern and instead use an accessor to get the address
of the test info mutex (defined only if MBEDTLS_TEST_MUTEX_USAGE is
defined, to hopefully stop more general usage)

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/tests/src/threading_helpers.c b/tests/src/threading_helpers.c
index 0894700..165e350 100644
--- a/tests/src/threading_helpers.c
+++ b/tests/src/threading_helpers.c
@@ -117,8 +117,6 @@
      * mbedtls_test_mutex_usage_check() will mark it as failed. */
 }
 
-extern mbedtls_threading_mutex_t mbedtls_test_info_mutex;
-
 static int mbedtls_test_mutex_can_test(mbedtls_threading_mutex_t *mutex)
 {
     /* If we attempt to run tests on this mutex then we are going to run into a
@@ -127,7 +125,7 @@
      * reporting that failure, as we already hold the mutex at that point.
      * 2. Given the 'global' position of the initialization and free of this
      * mutex, it will be shown as leaked on the first test run. */
-    if (mutex == &mbedtls_test_info_mutex) {
+    if (mutex == mbedtls_test_get_info_mutex()) {
         return 0;
     }