Unit tests for String append

During my debugging of the Line cuesettings, I found that string_getline was failing to grow a string (and therefore not actually adding the '--> <timestamp> <settings>' to the line. The reason this seemed to occur was because start lenght (9) + string length (23) == 32, which was the allocated length. Grow would return 32 correctly according to the rules of math, and there would actually be room in the string for this, but it would not copy the string in because the new length would be the same as alloc, even though there is technically room in the buffer for it.

Anyways, some unit tests for getline and append should help with problems like this.