test_db/test_db_copy_to: Fix hypothesis FailedHealthCheck error
With latest version of hypothesis, this test often raises the following error:
hypothesis.errors.FailedHealthCheck: Data generation is extremely slow ...
Setting the number of hypothesis examples for that test to a lower value makes the error goes away.
This should also fix that test execution in the swh-environment/tests Jenkins job which often fails.
See tests report on https://jenkins.softwareheritage.org/job/DENV/job/tests/
Migrated from D7842 (view on Phabricator)