Move base64 test interfaces into base64_internal.h

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/tests/suites/test_suite_base64.function b/tests/suites/test_suite_base64.function
index 052d1d0..e351ad8 100644
--- a/tests/suites/test_suite_base64.function
+++ b/tests/suites/test_suite_base64.function
@@ -1,5 +1,6 @@
 /* BEGIN_HEADER */
 #include "mbedtls/base64.h"
+#include "base64_internal.h"
 #include "constant_time_internal.h"
 #include <test/constant_flow.h>