Move constant_flow.h into the main library

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/tests/suites/test_suite_constant_time.function b/tests/suites/test_suite_constant_time.function
index bdd27f5..330dc6f 100644
--- a/tests/suites/test_suite_constant_time.function
+++ b/tests/suites/test_suite_constant_time.function
@@ -18,7 +18,7 @@
 #include <mbedtls/constant_time.h>
 #include <constant_time_internal.h>
 
-#include <test/constant_flow.h>
+#include "constant_flow.h"
 /* END_HEADER */
 
 /* BEGIN_CASE */