Bug 21800: Add tests for repeatable subfields

Bug 21800: Add tests for repeatable subfields

Test plan:
Do not apply the second patch [note 1].
Run t/db_dependent/Biblio/TransformKohaToMarc.t
Run t/db_dependent/Biblio/TransformMarcToKoha.t
Apply the second patch and run them again. Both tests should pass now.

Note 1: The TransformKohaToMarc test should fail with something like:
    #   Failed test 'Check 260e'
    #   at t/db_dependent/Biblio/TransformKohaToMarc.t line 60.
    #          got: 'A'
    #     expected: 'A | B'

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Ere Maijala <ere.maijala@helsinki.fi>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>