Bug 17304: Introduce C4::Matcher::_get_match_keys unit tests

Bug 17304: Introduce C4::Matcher::_get_match_keys unit tests

This function is used in several places, but it's behaviour is not covered by tests, thus trying to patch it can be problematic without introducing regressions.

This patch introduces unit tests for it, in the t/Matcher.t file.

To test:
- Apply the patch
- Run:
  $ prove t/Matcher.t
=> SUCCESS: Tests pass
- Sign off :-D

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer  <katrin.fischer@bsz-bw.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>