Adapt MessageFactoryTest::test_multipartAlternative to new replyAsHtml flag

TemplateParserJob got a new property replyAsHtml, by default false.
Only if set to true will the value of
TemplateParserSettings::replyUsingVisualFormat be respected.
That one though is false by default as well, only true if another test
set if before (happens on normal test runs), si it gets explicitly set
as well.