There's a well-known bit of constructed sample text that starts "Lorem ipsum"; it tries to approximate English letter frequencies, word length and word endings for experimenting with typography. There are other sorts of sample text that would be useful, though: for instance, font kerning relies on the spacing between letter pairs, so a sample text could be constructed which tried to contain every pair of letters you would find in English in the minimum amount of space. It shouldn't be too hard to construct some nonsense text that fits this requirement.