ve.dm.Converter.test: Use teardown to restore mw.config instead of ad-hoc

Follows-up I90273786ac186d.

QUnit.newMwEnvironment() is a utility provided by MediaWiki core
that returns an object with 'setup' and 'teardown' that automatically
provides a new sandboxed mw.config before each test and restores
the original after each test.

Change-Id: I24ac5d8be278df7070093fb84bcbe3720be18791