Adapt tests to the new QDateTime debug output behavior

Invalid QDateTimes (like the default-constructed ones) now output
"QDateTime(Invalid)".

Temporarily the test is disabled until qt5.git integration passes
such that the QDateTime debug operator stream becomes available
in the Qt CI.

Change-Id: I3c461e55f0300b8bc040761b8c181fe8abf63abe
Reviewed-by: Paolo Angelelli <paolo.angelelli@qt.io>