[devtools] add has_cache for postgres (closes #5739624) devtools stores info about existing dbs in the db handler, but in the case of postgresql that doesn't take into account the path to the cluster's datadir. Which means if we run two test modules (in the same test run), we'll create a "__default_empty_db__" for the first one, cache its existence, and then when moving on to the other module, believe the template already exists (but since the datadir depends on the test module's path, it does not). This patch is a bit of a kludge, and it would be better to make the cache key include enough data to not need this, but I'm not sure how to do that.