[devtools] Make PostgresTestDataBaseHandler multi-use friendly
Dont init the pgdb twice on the same datadir
- If two test case with the same apphome use a Postgres configuration, the
handler is initialised twice, so it has to check if the pgdb directory has
already been initialized.
- Work with the realpath of the pgdb
In some cases, the self.config.apphome will resolve symbolic links, but
not always.
It can result in an attempt to start twice the pg server for the same directory,
in the same test run... resulting in failure.
Closes #4875827