test: ignore the "freezing" & "thawing" intermediate states

When checking the unit state after `systemctl freeze|thaw` we can be
"too fast" and get the intermediate state (freezing/thawing) which we're
not interested in. Let's wait a bit and try to get the state again in
such cases to avoid unnecessary flakiness.

```
[   29.390203] testsuite-38.sh[218]: + state=thawing
[   29.390203] testsuite-38.sh[218]: + '[' thawing = running ']'
[   29.390203] testsuite-38.sh[218]: + echo 'error: unexpected freezer state, expected: running, actual: thawing'
[   29.390203] testsuite-38.sh[218]: error: unexpected freezer state, expected: running, actual: thawing
[   29.390203] testsuite-38.sh[218]: + exit 1
```