tests: ensure parserTestRunner restores $wgParser

tests: ensure parserTestRunner restores $wgParser

ParserTestRunner::addArticle() set a dummy $wgParser to ease test
running. However if doEditContent() fails, it would abort and never
restore $wgParser.

Make sure the restoration is done even on exception.

I caught that one while running parser tests with Scribunto and Cite. My
setup lacked a luastandalone which causes Scribunto to throw an
exception. When the Cite tests are run, the MessageCache parser ended up
using $wgParser = ParserTestMockParser;

Change-Id: I33a0a5c1f40f197405d40b7925cc043342e6757e