Fix memory allocation in ccm tests

The ccm tests were previously relying on unspecified behaviour in
the underlying implementation (i.e. that it rejects certain buffer
sizes without reading the buffer).

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/tests/suites/helpers.function b/tests/suites/helpers.function
index 80d47c8..aafcf5c 100644
--- a/tests/suites/helpers.function
+++ b/tests/suites/helpers.function
@@ -121,6 +121,7 @@
     TEST_ASSERT( ( expr1 ) == ( expr2 ) )
 
 /** Allocate memory dynamically and fail the test case if this fails.
+ * The allocated memory will be filled with zeros.
  *
  * You must set \p pointer to \c NULL before calling this macro and
  * put `mbedtls_free( pointer )` in the test's cleanup code.