Write tests for confirming all meta tags output correctly

WIP. Almost there, at least for the base module.

Fixed the base module's tests.

Renamed one of the callbacks, simplified the canonical & shortlink tests.

This fixes up twitter cards and validation tests, and extends the internal API a little.

This fixes the output of Google Plus tags, and fixes tests for Google Plus and Open Graph.

Woo! All green! Committed.

This needs to be backported to 7.x-1.x.

Initial port to D7 of the functionality for the main module, submodules will come next.

A PHP 5.6-ism slipped in by accident.

Fixed tests for the AppLinks, Dublin Core and Facebook tags.

This adds tests for favicons, and fixes a bug where the core shortcut icon was still being output in certain circumstances.

Tests for the metatag_google_cse and metatag_google_plus submodules. This also should fix #2832943: Itemprop is empty?.

Fixed the metatag_hreflang tests.

Mobile.

Fixed OG, OG Products and Verification. Last stop - Twitter Cards.

And Twitter.

So MetatagMobileTagsTest fails on the testbot, but it works locally. OTOH the MetatagCoreXSSTest fails locally X-)

I reran the tests locally using PHP 5.3 instead of my usual 5.6, and MetatagMobileTagsTest still works.

A minor tweak to confirm that the test page was actually loaded.

Simplify the handling of errors when the tag doesn't output properly.

This will fail spectacularly, but it'll help work out what the problem is.

So the root of the problem was that there were test classes with duplicate names. Duh.

This should cover it.

That also fixes the ios-app and android-app alternate tags which were using rel="alternative" instead of rel="alternate". Doh.

Committed! Woot!

We'll do some additional follow-up work in #2834411: Resolve @todo items in tests.