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/linux/linux.c b/test/linux/linux.c
index beffe8e..d98d412 100644
--- a/test/linux/linux.c
+++ b/test/linux/linux.c
@@ -25,7 +25,7 @@
 #include "hf/dlog.h"
 #include "hf/socket.h"
 
-#include "hftest.h"
+#include "test/hftest.h"
 #include <sys/socket.h>
 #include <sys/syscall.h>
 #include <sys/types.h>