tests: Fix path of yosys invocation in xprop tests

For now xprop test failures are still expected and ignored, but without
this change, they did not even run unless the yosys build was in path.

tests/xprop/test.py

@@ -47,7 +47,7 @@ if "clean" in steps:
 
 
 def yosys(command):
-    subprocess.check_call(["yosys", "-Qp", command])
+    subprocess.check_call(["../../../yosys", "-Qp", command])
 
 def remove(file):
     try: