tests: fix assignment typo, should be comparison

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
diff --git a/tests/surface-global-test.c b/tests/surface-global-test.c
index dbb8c8d..5908f60 100644
--- a/tests/surface-global-test.c
+++ b/tests/surface-global-test.c
@@ -55,7 +55,7 @@
 
 	weston_surface_from_global_fixed(surface, wl_fixed_from_int(21),
 					 wl_fixed_from_int(100), &fx, &fy);
-	assert(fx = wl_fixed_from_int(16) && fy == wl_fixed_from_int(90));
+	assert(fx == wl_fixed_from_int(16) && fy == wl_fixed_from_int(90));
 
 	weston_surface_from_global(surface, 0, 0, &ix, &iy);
 	assert(ix == -5 && iy == -10);