Move hftest headers to a test inc directory.

Other common test headers can also go under this directory, rather
than having shared 'inc' directories scattered around the test source
tree.

Change-Id: I990bdc11ad08bd44377ac2e064c566f552ff22b5
diff --git a/test/arch/mm_test.c b/test/arch/mm_test.c
index e2d1eb8..bf0c186 100644
--- a/test/arch/mm_test.c
+++ b/test/arch/mm_test.c
@@ -18,7 +18,7 @@
 
 #include "hf/arch/mm.h"
 
-#include "hftest.h"
+#include "test/hftest.h"
 
 /** There must be at least two levels in the page table.  */
 #define MAX_LEVEL_LOWER_BOUND 1