test: fix flaky vinyl/gh test failure

The commit fixes the following test failure:

```
[005] vinyl/gh.test.lua                                               [ fail ]
[005]
[005] Test failed! Result content mismatch:
[005] --- vinyl/gh.result	Mon Dec 13 15:03:45 2021
[005] +++ /root/actions-runner/_work/tarantool/tarantool/test/var/rejects/vinyl/gh.reject	Fri Dec 17 10:41:24 2021
[005] @@ -716,7 +716,7 @@
[005]  ...
[005]  test_run:wait_cond(function() return finished == 2 end)
[005]  ---
[005] -- true
[005] +- false
[005]  ...
[005]  s:drop()
[005]  ---
```

The reason of the failure is that the fiber doing checkpoints fails,
because a checkpoint may be already running by the checkpoint daemon.
Invoke box.snapshot() under pcall to make the test more robust.

Part of #5141

(cherry picked from commit cc6c328d)