Fix link target and retina thumbnails for manual thumbnails. (Bug 45405.)

Fix link target and retina thumbnails for manual thumbnails. (Bug 45405.)

The output for [[Image:Bad.jpg|thumb=Foobar.jpg|Title]] used to be:

<div class="thumb tright"><div class="thumbinner" style="width:1943px;"><a
href="/wiki/File:Foobar.jpg" class="image"><img alt=""
src="http://example.com/images/3/3a/Foobar.jpg" width="1941" height="220"
class="thumbimage" srcset="http://example.com/images/0/09/Bad.jpg 1.5x,
http://example.com/images/0/09/Bad.jpg 2x" /></a>  <div
class="thumbcaption"><div class="magnify"><a href="/wiki/File:Bad.jpg"
class="internal" title="Enlarge"><img
src="/skins/common/images/magnify-clip.png" width="15" height="11" alt=""
/></a></div>Title</div></div></div>

Note that the target of the <a> is the thumb, not the original image,
and that the srcset is loading the full resolution version of Bad.jpg.

The attached patches fix the link target and srcset issues
(suppressing the srcset when a manual thumb is used).  It also adds a
new "Thumb.png" pseudo-file to the parserTests so that we can write
new tests documenting how manual thumbnails are expected to work,
and adds the 'php' option to the thumbnail tests (since the Parsoid
parser generates different output).

Change-Id: I5be80bfce855b85f9debf3ef1776b877d1f84b9f