qunit: Update testrunner against changes in MediaWiki core
The result of 'QUnit.newMwEnvironment' is only for passing to QUnit.module(),
not to call methods on directly.
Instead of wrapping 'QUnit.newMwEnvironment()' and returning a plain object
with only setup/teardown, use 'QUnit.newMwEnvironment' directly and pass
setup/teardown to it.
- Other properties are preserved instead of silently dropped.
- Core changes from setup/teardown to beforeEach/afterEach will continue to work because Wikibase won't rely on what is returned from it.
- Don't use the 'new' operator, since this isn't an instantiable class, it's a static method that returns a plain object.
- Don't overwrite mw.config.values, instead call mw.config.set().