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/Makefile b/tests/Makefile
index aba002b..bc88e82 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -104,6 +104,7 @@
 	echo "  CC    $<"
 	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) $<	$(LOCAL_LDFLAGS) $(LDFLAGS) -o $@
 
+$(filter test_suite_psa_crypto%, $(BINARIES)): psa_helpers.function
 
 clean:
 ifndef WINDOWS