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/power_mgmt.c b/test/hftest/power_mgmt.c
index 2daba31..4c651a6 100644
--- a/test/hftest/power_mgmt.c
+++ b/test/hftest/power_mgmt.c
@@ -20,7 +20,7 @@
 
 #include "hf/spinlock.h"
 
-#include "hftest.h"
+#include "test/hftest.h"
 
 struct cpu_start_state {
 	void (*entry)(uintptr_t arg);