Bug 35989: (QA follow-up) Add test and limit variable scope

Bug 35989: (QA follow-up) Add test and limit variable scope

Before this patch if a record had a 751 and a 781 you could have fields repeated.
This patch reduces the scope of the fields to subdivision variable as it is only used in processing 7xx
fields and should not be shared between fields.

I also add unit tests

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Aleisha Amohia <aleishaamohia@hotmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 95f2aecc1b57b7c4bb8326f9f2fc9366a042a9c1)
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 4ca639a5f64347db02416c881d15ff0e53ee4cb7)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>