Fix SearchEngineTest when $wgSearchType is set to non-default

Fix SearchEngineTest when $wgSearchType is set to non-default

$wgSearchType needs to be controlled for at time the pages get created.
This patch sets the $wgSearchType global.

Also, the 'singleton' is removed from SearchEngine::getSearchTypes, which
appears to provide little or no benefit while binding all tests to the
global variable.

SearchEngine::getSearchTypes() gets called in SearchUpdate which is
run on page edits. This means the first page edit in the entire
test suite causes the global variable to be set and subsequent tests
cannot override it.

These changes allow SearchEngineTest to pass, even if one has CirrusSearch
or other such extensions enabled.

Change-Id: I39050da8659dc69db31125f469f494a5fb4b8fca