tests: mock parser tests file access

I eventually got tired of our parser tests creating and deleting fixture
files over and over.  This patch mock the files in memory and just
expose the file metadata which is all we need for parser tests.

The mocked classes are under /tests/phpunit/mocks/ and respect the
hierarchy of /includes/.

The wiki.png and headbg.jpg files are still copied on each test :/

Change-Id: Iccdff67222e66d48d01dd1596d09df2ea24b8c2a