Fix parallel testing (issue #48547) - #48560

Fixes 2 bugs in parallel testing.

The first bug was related to changes made in rails#45450 which meant that we were
no longer replacing the connection in parallel testing because the
config object is equal (we simply merge a new db name but the object id
of the config stays the same). This bug only manifested in mysql and
sqlite3 interestingly. It would fail on the internal metadata tables
because they were missing in the schema version check.

To fix this I introduced a `clobber: true` kwarg onto the connection
handler that allows us to bypass the functionality that won't make a new
connection if the config is the same. This is an easy way to fall back
to the old behavior from before this change. I only added `clobber`
to the `reconstruct_from_schema` call because we need to actually
replace the connection for these. It's not safe to add everywhere since
we don't always want to replace the connection.

After implementing this fix I was still seeing failures in the mysql
demo app I made due to the fact that `purge` was not re-establishing the
connection to a config that had a database defined. Neither sqlite3 or
postgresql were missing this.

I added a test for mysql2 so we don't have regressions in the future. I
think this was missed because sqlite3 only demonstrates the bug if it
was never successful on that worker and postgresql was fine.

Fixes rails#48547