Bug 29403: Fix unit test for bug 29718

Bug 29403: Fix unit test for bug 29718

Including a 'T' is actually valid for ISO date format, I was being
naughty and writing the test to follow our code rather than writing the
test to conform to the ISO spec first time around. Now we are properly
supporting 'T' in ISO dates with bug 29718, we need to update the test
here to properly reflect an invalid ISO date string.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
The commit message is misleading here. Look at the Z in the code changes.
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>