FIX: Fixed testing of theme switching.

The previous theme test relied on the existence of the tutorial theme, which has been removed.
I'ved added a couple of behat rules for scaffolding fake themes, and altered the test to use these.
FIX: Added proper state-restoration code to the ThemeContext:
Minor improvement of theme switching test performance.
Merge pull request #937 from sminnee/fix-theme-test

FIX: Fixed testing of theme switching.