Bug 30287: (follow-up) Unit test for html_content

Bug 30287: (follow-up) Unit test for html_content

This patch adds a unit test for the new `html_content` method introduced
to Koha::Notice::Template in the previous patch.

Test
1) Run the new unit test ;P

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit c260f4a374c3e3fcb335e997842ffb2fa1bd444a)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>