tests: Don't set wgStyleSheetPath in addition to wgStylePath

Because 1) `$wgStyleSheetPath = &$wgStylePath;` in default
settings, so setting one sets the other. No need to set both
and 2) in wmf-branches this variable is unset, thus this
caused an E_NOTICE internally when Test::setMwGlobals is
trying to access it to preserve the current value,
and 3) wgStyleSheetPath is deprecated.

Follows-up I1362932db223.

Change-Id: Ibd3f28e460fef995f68dfe1292d25fb75950dcf5