Bug 27007: Don't call GetMarcSubfieldStructure with unsafe in tests

Bug 27007: Don't call GetMarcSubfieldStructure with unsafe in tests

It does not feel correct to call GetMarcSubfieldStructure with the unsafe flag set in tests:
   my $mss = C4::Biblio::GetMarcSubfieldStructure( '', { unsafe => 1 } );

Some tests are failing is run twice

Use of uninitialized value $code_wanted in string eq at /usr/share/perl5/MARC/Field.pm line 314.
[x~10]
Use of uninitialized value $code_wanted in string eq at /usr/share/perl5/MARC/Field.pm line 314.

    #   Failed test 'Value is mapped correctly for column biblionumber'
    #   at t/db_dependent/Koha/Item.t line 115.
    #          got: undef
    #     expected: '483'
[etc.]

Test plan:
Run the tests twice, without the patch it fails, with the patch it
passes

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>