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/hftest_socket.c b/test/linux/hftest_socket.c
index 1e55551..6c9a244 100644
--- a/test/linux/hftest_socket.c
+++ b/test/linux/hftest_socket.c
@@ -24,7 +24,7 @@
 #include "vmapi/hf/call.h"
 #include "vmapi/hf/transport.h"
 
-#include "hftest.h"
+#include "test/hftest.h"
 
 alignas(4096) uint8_t kstack[4096];