Move constant_flow.h into the main library

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/tests/src/helpers.c b/tests/src/helpers.c
index 30fd362..1062560 100644
--- a/tests/src/helpers.c
+++ b/tests/src/helpers.c
@@ -15,7 +15,7 @@
  *  limitations under the License.
  */
 
-#include <test/constant_flow.h>
+#include "constant_flow.h"
 #include <test/helpers.h>
 #include <test/macros.h>
 #include <string.h>