Avoid timeouts in the notify system test

There were two problems in the notify system test when it waited for log messages to appear: the shellcheck refactoring introduced a call to wait_for_log with a regex, but wait_for_log only supports fixed strings, so it always ran for the full 45 second timeout; and the new test to ensure that notify messages time out failed to reset the nextpart pointer, so if the notify messages timed out before the test ran, it would fail to see them.

This change adds a wait_for_log_re helper that matches a regex, and uses it where appropriate in the notify system test, which stops the test from waiting longer than necessary; and it resets the nextpart pointer so that the notify timeout test works reliably.

Closes #3275 (closed)