Bug 27554: Clarify and add tests for update_category_to

Bug 27554: Clarify and add tests for update_category_to

This patch adds a comment to the existing tests, clarifying what they test, and verifying the expected patron is affected. The
original test should be broken by 26307 when the behvaiour is prevented

I additionally add a test for 'too old'

To test:
1 - Apply patch
2 - Prove tests, they should pass
3 - Verify the comments make sense

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
JD amended patch: behvaiour ==> behaviour
and add more info to the commit title

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>