test: Fix MAC address calculation

The bit masking was using overzealous shift widths, resulting in "always
zero" second and third components.

While at it, improve readability by first shifting and then masking.

Found by Coverity (CID #15239).

test/common/testvm.py

@@ -853,7 +853,7 @@ def interface(self, number=None):
         self.hostnet += 1
         result = {
             "number": self.offset + number,
-            "mac": '52:54:01:{:02x}:{:02x}:{:02x}'.format(mac & 0xff0000 >> 24, mac & 0xff00 >> 16, mac & 0xff),
+            "mac": '52:54:01:{:02x}:{:02x}:{:02x}'.format((mac >> 16) & 0xff, (mac >> 8) & 0xff, mac & 0xff),
             "name": "m{0}.cockpit.lan".format(mac),
             "mcast": self.network,
             "hostnet": "hostnet{0}".format(hostnet)