This adds a "test.asserts" module that currently just provides a
convenient way to assert on the content of warnings. By default all
tests will assert that no warnings are given.
Before the XDG variables would be set from the user's environment, if
available. This would break some tests.
With this change the tests should be fully deterministic.
Fixes#1190
Add 'services.lieer', which generates systemd timer and service units
to synchronize a Gmail account with lieer. Per-account configuration
lives in 'accounts.email.accounts.<name>.lieer.sync'.
