Move declarations of testing-only base64 functions to their own header

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_base64.function b/tests/suites/test_suite_base64.function
index c054895..d0e1167 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_invasive.h"
 #include <test/constant_flow.h>
 /* END_HEADER */