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_pk.function b/tests/suites/test_suite_pk.function
index fd923c2..0e02c3e 100644
--- a/tests/suites/test_suite_pk.function
+++ b/tests/suites/test_suite_pk.function
@@ -10,6 +10,11 @@
 #include <limits.h>
 #include <stdint.h>
 
+#if defined(MBEDTLS_USE_PSA_CRYPTO)
+#include "mbedtls/psa_util.h"
+#include "psa_helpers.function"
+#endif
+
 static int rnd_std_rand( void *rng_state, unsigned char *output, size_t len );
 
 #define RSA_KEY_SIZE 512
@@ -67,8 +72,6 @@
 
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
 
-#include "mbedtls/psa_util.h"
-
 /*
  * Generate a key using PSA and return a handle to that key,
  * or 0 if the key generation failed.