Keep test text when font changes

[markhdavid]

I'd like it to keep the test text when the font changes, at least as an option, or have a way to change what the default text is. As it is, if I have some test text I want to test on N fonts, I have to replace the test text N times, which is tedious and somewhat error prone.

[justvanrossum]

Related to, or perhaps duplicate of #40.