test: fix flaky vinyl/gc test

The commit fixes the following test failure:

```
[013] vinyl/gc.test.lua                                               [ fail ]
[013]
[013] Test failed! Result content mismatch:
[013] --- vinyl/gc.result	Fri Dec 24 12:27:33 2021
[013] +++ /build/usr/src/debug/tarantool-2.10.0~beta2.18.dev/test/var/rejects/vinyl/gc.reject	Thu Dec 30 10:29:29 2021
[013] @@ -102,7 +102,7 @@
[013]  ...
[013]  check_files_number(2)
[013]  ---
[013] -- true
[013] +- null
[013]  ...
[013]  -- All records should have been purged from the log by now
[013]  -- so we should only keep the previous log file.
```

The reason of the failure is that vylog files are deleted asynchronously
(`box.snapshot()` doesn't wait for `unlink` to complete) since commit
8e429f4b ("wal: remove old xlog files
asynchronously"). So to fix the test, we just need to make the test wait
for garbage collection to complete.

Follow-up #5383

(cherry picked from commit cd9fd77e)