Move the ARRAY_LENGTH definition to common.h

Reuse it in the library and tests.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/tests/src/psa_crypto_helpers.c b/tests/src/psa_crypto_helpers.c
index 77c2f89..8f58d4d 100644
--- a/tests/src/psa_crypto_helpers.c
+++ b/tests/src/psa_crypto_helpers.c
@@ -24,6 +24,7 @@
 #include <test/macros.h>
 #include <psa_crypto_slot_management.h>
 #include <test/psa_crypto_helpers.h>
+#include "common.h"
 
 #if defined(MBEDTLS_PSA_CRYPTO_C)