parserTests: Use "fallback" skin unless otherwise specified

parserTests: Use "fallback" skin unless otherwise specified

ParserOutput::getText() depends on the current skin to format edit
section links, which some (e.g. MinervaNeue) have customized. This
causes parser tests to fail when they expect the default edit section
link format.

Default to always using the built-in fallback skin when parsing, but
allow it to be overridden with a "skin=..." option.

We also have to override $wgOut in addition to the main RequestContext
since that's what ParserOutput uses.

Bug: T170880
Change-Id: Ib7f0bd15dd0a9255e1e5140907e800478b658b92