When the programdata path is missing, ensure that we pass down the
`GIT_ENOTFOUND` error to the caller instead of converting it to a
generic `-1`.

Now that we've split the notion of the home directory from the global
configuration store, our tests should use the appropriate one based on
what they're doing.

Like we want to separate libgit2 and utility source code, we want to
separate libgit2 and utility tests.  Start by moving all the tests into
libgit2.