XmlReporter: add information about rng-seed

Xml result of reported will now contain value of rng-seed in case it
is not zero.
The value will be stored in element Randomness and it's attribute seed.

Relates to #1402

include/reporters/catch_reporter_xml.cpp

@@ -55,6 +55,9 @@ namespace Catch {
         m_xml.startElement( "Catch" );
         if( !m_config->name().empty() )
             m_xml.writeAttribute( "name", m_config->name() );
+        if( m_config->rngSeed() != 0 )
+            m_xml.scopedElement( "Randomness" )
+                .writeAttribute( "seed", m_config->rngSeed() );
     }
 
     void XmlReporter::testGroupStarting( GroupInfo const& groupInfo ) {