We had just 3 tests remaining in test_io.py. This commit moves them to
their modules. So we have:
* test_graceful_shutdown.py
* test_zero_downtime.py
* test_pg_internal.py
Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>
(cherry picked from commit 02d83d1c01)