Fix timeout in `cross-origin-top-navigation-with-user-activation...`

[jonathan-j-lee]

The test drops testdriver commands and times out in all browsers because:

1. The top document doesn't embed testharness*.js.
2. The opened document needs to set_test_context() to send testdriver commands to its parent.

Also, use variants to merge the test file with the very similar "without activation" case.