Make wfScript()/mw.util.wikiScript() respect $wgScript/$wgLoadScript

Make wfScript()/mw.util.wikiScript() respect $wgScript/$wgLoadScript

mw.util.wikiScript() and mw.util.wikiScript( 'index' ) now return the
value of $wgScript rather than building the path to index.php manually;
the latter is incorrect if $wgScript is customized. Similarly,
mw.util.wikiScript( 'load' ) now uses $wgLoadScript.

Same for wfScript(), wfScript( 'index' ) and wfScript( 'load' )

The qunit tests already had a test case for wgScript, but wgScript was
set in a way that didn't trigger the bug. Changed it to use customized
wgScript and wgLoadScript values and added test cases for 'index' and
'load'

This fixes bug 39103 (wfScript/mw.util.wikiScript do not respect custom
$wgScript/$wgLoadScript) as well as bug 39102 ("Entry points" on
Special:Version do not respect custom $wgScript/$wgLoadScript)

Change-Id: I5c9e82849e314dc7a87f8ae91150cc412b4011cf