Don't leak memory in tests

[isbadawi]

It would be nice to get useful output out of LSan in the tests, but there's a lot of noise because the tests themselves allocate things without freeing. Some leaks could maybe be suppressed (e.g. LSAN_OPTIONS=suppressions=<(echo "leak:test_*__initialize")) but it would be better to fix.

Some structs like editor, buffer, gapbuf don't have free functions because they stay around for the life of the program (for now), so those would have to be added.