Improved tests for statestore - redis

[akremsa]

What this PR does / Why we need it:
I've noticed that redis.go has low unit test coverage and decided to improve that.

Before:

go test -coverprofile=before.out
PASS
coverage: 52.3% of statements
ok  	open-match.dev/open-match/internal/statestore	1.394s

After:

go test -coverprofile=after.out
PASS
coverage: 84.8% of statements
ok  	open-match.dev/open-match/internal/statestore	1.007s

[akremsa]

@Laremere could you please take a look?

[aLekSer]

Fine with me.

[Laremere]

There was already reasonably good coverage of this through the e2e tests (unfortunately coverage doesn't work across packages.) This seems like enough of a positive, and don't want to waste your work, so approving.
I think in the future if there are instances where these tests need to change and coverage is handled by e2e tests, I'll be fine with PR authors either fixing this test, or removing test cases here (again, as long as it's also covered well by e2e.)

[Laremere]

Thanks for the PR!