tests: fix make check for out-of-tree builds

It was failing with missing include files.

While here, destroy the ugly "../src/..." include
paths used in the tests that was just hacking around
this problem in the Makefile:

sed -i s/..\\/src\\/// tests/*.c

tests/fixed-benchmark.c

@@ -25,7 +25,7 @@
 #include <string.h>
 #include <time.h>
 #include <assert.h>
-#include "../src/wayland-private.h"
+#include "wayland-private.h"
 
 volatile double global_d;