[9.18] fix: test: Fix the .hypothesis directory to bin/tests/system/.hypothesis

Previously the location of the .hypothesis directory would depend on the
current working directory when running pytest.

Set the HYPOTHESIS_STORAGE_DIRECTORY explicitly.

Closes #5424

Backport of MR !10825

Merge branch 'backport-5424-hypothesis-artifacts-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10829