I want to be able to easily test the C API on actual data in an OSTree
repo. The shell `libtest.sh` has code to generate it. Bridge the two
worlds by introducing a little `libostreetest` library which has a C
API which spawns a shell that runs things in `libtest.sh`.
Yes, this is about as beautiful as it sounds, which is to say, it's
not. But it works!
Note while we were here, I realized we were actually now creating
*two* tmpdirs per test in `make check` because the tap driver was
already doing that. Unify it so we know the C code can rely on it.
Colin Walters