Bug 19693: Add test sub to Merge.t

Bug 19693: Add test sub to Merge.t

In those rare cases when authorities are updated by an external agency (or
even internally, by reviewing and correcting an exported authority file)
when the heading tag will be changed (seems odd but happens:

111 Congress ==> 110 Corporate body.Congress ;
100 Person ==> 110 Corporate body (a company named with person's name ;
151 City--object ==> 150 Object (city) etc.)

and then the authority record in Koha database will be updated with
bulkmarcimport or by calling directly ModAuthority from a custom script,
the merge function "doesn't know" that the change to the authority type
has been made and, consequently, doesn't adequately change the tag in
related fields in biblio records (as it would if two different records
with different authtypecode were merged with Koha interface).

This is because at the moment when merge function is being called
by ModAuthority:
Koha::Authority::Types->find($autfrom->authtypecode)
Koha::Authority::Types->find($authto->authtypecode)
both have the same value (because $mergefrom == $mergeto).

Therefore in case when $mergefrom == $mergeto and the heading tag changes,
$authtypefrom and $authfrom calculated in merge in the ordinar way are
misleading and should not be taken unto consideration.

Test plan:
==========

1. run t/db_dependent/Authority/Merge.t
2. you should see problems in "Test update A with modified heading tag
   (changing authtype)"
3. apply the patch
4. run t/db_dependent/Authority/Merge.t again
5. the test should pass

Alternatively:

1. have an authority record used in biblio;
   export it to file;
   change 1XX heading tag to a different (but reasonable) value
   and possibly change also the content of the heading
   (one can delete also 942 but it doesn't matter);
   make bulkmarcimport.pl -a -update -file <modified_auth_file> and
   see that the tag in biblio record has not been changed (whereas
   the type of authority record did change);

2. make orders in database (so that the authority type and the tag of
   the field in biblio record correspond); apply the patch;

3. repeat the test from 1.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 4d75a9b3b786d75e1afb5f66e51f48e9e503d454)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 1ac516addb95cfaf7778a02387722edfb2b21a38)
Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>