Make the selectedThread URL test use the latest URL version.
Make the selectedThread URL test use the latest URL version.

Also add a test for multi-thread selection.

I had to add pids to the threads because otherwise thread 0 would not have been
visible by default. It would have been overwritten by thread 2 because we currently
don't handle multiple GeckoMain threads with the same pid very well.

mstange/cleopatra,mstange/cleopatra