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_crypto.function b/tests/suites/test_suite_psa_crypto.function
index aaa3189..2e2606f 100644
--- a/tests/suites/test_suite_psa_crypto.function
+++ b/tests/suites/test_suite_psa_crypto.function
@@ -1,15 +1,11 @@
 /* BEGIN_HEADER */
 #include <stdint.h>
 
-#if defined(MBEDTLS_PSA_CRYPTO_SPM)
-#include "spm/psa_defs.h"
-#endif
-
 #include "mbedtls/asn1.h"
 #include "mbedtls/asn1write.h"
 #include "mbedtls/oid.h"
 
-#include "psa/crypto.h"
+#include "psa_helpers.function"
 
 /** An invalid export length that will never be set by psa_export_key(). */
 static const size_t INVALID_EXPORT_LENGTH = ~0U;