Bug 19227: Reduce the number of tests run by 00-merge-conflict-markers.t

Bug 19227: Reduce the number of tests run by 00-merge-conflict-markers.t

The number of tests on jenkins is sometimes confusing:

https://jenkins.koha-community.org/job/Koha_Master_D8/lastCompletedBuild/testReport/(root)/t_00_merge_conflict_markers_t/

shows that 00-merge-conflict-markers.t ran 10,751 tests, 124 less than
the previous run. However 124 files have not been removed from the
codebase!

I suggest to count only 1 test for all files.

Moreover files from blib and cover_db are counted, they should be
excluded.

Test plan:
  prove t/00-merge-conflict-markers.t
must return green

echo ">>>>>>>" >> mainpage.pl
and run the test again
It should now fail

Signed-off-by: Magnus Enger <magnus@libriotech.no>
Works as advertised.
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>