tests: add default timeout

Set a timeout at two minutes to catch tests that fail to boot.

tests/lib/testing-python.nix

@@ -58,6 +58,8 @@ rec {
 
         LOGFILE=/dev/null tests='exec(os.environ["testScript"])' ${driver}/bin/nixos-test-driver
       '';
+
+      meta = { timeout = 120; } // driver.meta or { };
     };
 
   defaultTestScript = ''