Improve selector_to_xpath tests on pseudo-elements

[dangra]

This PR addresses this comments d29ac49#commitcomment-4358942

[SimonSapin]

Thanks for contributing. Could you fix Python 2.5 compat and move this to test_pseudo_elements?

[dangra]

let me know if you prefer a solution like this http://stackoverflow.com/questions/11285313/try-except-as-error-in-python-2-5-python-3-x instead of asserting only the exception type.

I will squash into a sigle commit once it passes tests on all platforms.

[SimonSapin]

I think assertRaises is enough, that was in case you wanted to keep testing the message.

Looks good, thanks!