[sailfish-browser] Add unit test cases for suffling tabs

Aim is to automate a test where a WebPage stops responding. Added
cases do not reproduce that error yet.

Requires: https://github.com/sailfishos/sailfish-browser/pull/208
These two methods seem to do exactly the same work, but implemented with two different approaches.

I'd probably simplify it a bit: turn it into a list comprehension with a filter. And the list comprehension part could be shared between the methods.