Bug 987946 - [unittest] test_cold_launch.py failing

The test is failing because the correct app url is now not available until
shortly after launching.
This patch changes the test so it waits for the correct app url, rather than
just immediately asserting that it is as expected.
Merge pull request #17604 from bobsilverberg/cold_launch

Bug 987946 - [unittest] test_cold_launch.py failing