Add a temporary mutex around the calls to sqldb.NewTestSqliteDB to
ensure that only a single thread can access it at a time. This is a
temporary work around that can be removed once this race condition in
the sqlite repo has been resolved:
https://gitlab.com/cznic/sqlite/-/issues/180
This commit extracts the InvoiceDB construction from all invoice and
registry tests such that we can later on run subtests with multiple
backends without needing to use tags.
