Fixing bug in dbupdate where it would explode on plugin that is missing MODELS or MIGRATIONS

The reason it blew up was because in the latter caught exception, it
wouldn't set models/migrations to an empty set, so it would actually
use the previous run's models/migrations!  That's what we get for
"leaky" variables on python for loops :)

This commit sponsored by Pascal Diogo Antunes.  Thank you!
Always activate testing in every test module ever.

Kind of a dorky way to implement this, but...
Always activate testing using pytest fixture.

This is possibly the cleaner approach to the problem solved
in the previous commit.

The previous commit makes sure, that testing is enabled at
import time. While this one enables it just before the
individual test is run. So this one takes a bit more CPU.

Maybe revert the previous commit?

References: 0536306048daa0970d2e43411ba2a9bf073e570e
Reverting "Always activate testing in every test module ever."
Revert "Always activate testing in every test module ever."

This reverts commit 0536306048daa0970d2e43411ba2a9bf073e570e.
Merge remote-tracking branch 'refs/remotes/elrond/misc/pytest_enable_testing'