WebSubmit: optional pdftk regression tests

• Disables WebSubmit stamping related regression tests in case pdftk is not available.

Reported-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>