Create PSA-specific helper function file

Create a specific file for helper functions that are related to the
PSA API. The reason for a separate file is so that it can include
<psa/crypto.h>, without forcing this header inclusion into every test
suite. In this commit, psa_helpers.function doesn't need psa/crypto.h
yet, but this will be the case in a subsequent commit.

Move PSA_ASSERT to psa_helpers.function, since that's the sort of
things it's for.

Include "psa_helpers.function" from the PSA crypto tests.

In the ITS test, don't include "psa_helpers". The ITS tests are
meant to stand alone from the rest of the library.
diff --git a/tests/suites/test_suite_psa_its.function b/tests/suites/test_suite_psa_its.function
index 867f64f..873e1a2 100644
--- a/tests/suites/test_suite_psa_its.function
+++ b/tests/suites/test_suite_psa_its.function
@@ -1,6 +1,8 @@
 /* BEGIN_HEADER */
 #include "../library/psa_crypto_its.h"
 
+#define PSA_ASSERT( expr ) TEST_EQUAL( ( expr ), PSA_SUCCESS )
+
 /* Internal definitions of the implementation, copied for the sake of
  * some of the tests and of the cleanup code. */
 #define PSA_ITS_STORAGE_PREFIX ""