fix: test: Handle example3.db being modified in upforwd system test

The zone file for example3 (ns1/example3.db) can be modified in the
upforwd test as example3 is updated as part of the test.  Whether
the zone is written out or not by the end of the test is timing
dependent.  Rename ns1/example3.db to ns1/example3.db.in and copy it to
ns1/example3.db in setup so we don't trigger post test changes checks.

Closes #5180

Merge branch '5180-create-example3-in-setup' into 'main'

See merge request isc-projects/bind9!10160