nixos/test: some test fixes in succession of #125992

+1
nixos/tests/postgresql.nix
···
with subtest("Postgresql survives restart (bug #1735)"):
machine.shutdown()
time.sleep(2)
machine.start()
machine.wait_for_unit("postgresql")
···
with subtest("Postgresql survives restart (bug #1735)"):
machine.shutdown()
+
import time
time.sleep(2)
machine.start()
machine.wait_for_unit("postgresql")
+1
nixos/tests/turbovnc-headless-server.nix
···
else:
if check_success():
return
time.sleep(retry_sleep)
if not check_success():
···
else:
if check_success():
return
+
import time
time.sleep(retry_sleep)
if not check_success():