Adds XmlLayoutFileValidityTest

In case of an issue the test will output something like:
1) Skins\Tests\Chameleon\XmlLayoutFileValidityTest::testXmlValidityOfLayoutFiles
DOMDocument::load(): Opening and ending tag mismatch: component line 10 and structure in file:///../../layouts/fixedhead.xml, line: 46

Change-Id: I008c5056f6918f97162d2b0cd1911a314237a72b