lib: reject relative paths in n_d_{create,open}_verbose

lib: reject relative paths in n_d_{create,open}_verbose

There are many places in the notmuch code where the path is assumed to be absolute. If someone (TM) wants a project, one could remove these assumptions. In the mean time, prevent users from shooting themselves in the foot.

Update test suite mark tests for this error as no longer broken, and
also convert some tests that used relative paths for nonexistent
directories.