Cross-database testing

[fluffynuts]

Log4Net can log to multiple backends, including multiple database backends. I've seen a situation before where a patch to improve performance for postgresql negatively impacted mssql users (something to do with parameter initialisation). To this end, we should have a github action which runs log4net against multiple database backends and verifies that the results are correct for both, perhaps also recording timings and referring to expected timings to raise flags about any massive change in runtime.