arti: Write the test config atomically.

The flakiness from the watch_single_file test was caused by the race between the config write, and the fake SIGHUP "signal" sent on sighup_tx: sometimes, the sighup would get handled between creating the config file and writing its contents. In those cases, the config received in TestModule::reconfigure would be Default::default(), which caused an assertion to fail (because the test is expecting to receive the ArtiConfig it wrote to disk).

Closes #1549 (closed)