Move the ARRAY_LENGTH macro to the common helpers file

tests/suites/helpers.function

@@ -150,6 +150,9 @@ typedef struct data_tag
     mbedtls_exit( 1 );                                             \
 }
 
+/** Return the number of elements of a static or stack array. */
+#define ARRAY_LENGTH( array )                   \
+    ( sizeof( array ) / sizeof( *( array ) ) )
 /*
  * 32-bit integer manipulation macros (big endian)
  */