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/hftest/common.c b/test/hftest/common.c
index 7a1cea0..877f1db 100644
--- a/test/hftest/common.c
+++ b/test/hftest/common.c
@@ -21,8 +21,8 @@
 #include "hf/memiter.h"
 #include "hf/std.h"
 
-#include "hftest.h"
 #include "hftest_common.h"
+#include "test/hftest.h"
 
 HFTEST_ENABLE();