test(store): ensure busy manager goroutine is finished (#15788)

## Description

Without this change, there were intermittent failures due to a non-empty t.TempDir() directory at the end of a test:

```console
$ go test  -run=LoadChunk$ ./snapshots/ -count=220 -failfast 
--- FAIL: TestManager_LoadChunk (0.01s)
    testing.go:1225: TempDir RemoveAll cleanup: unlinkat /var/folders/.../T/TestManager_LoadChunk1472891739/001: directory not empty
FAIL
FAIL    cosmossdk.io/store/snapshots    0.177s
```

With this change, I can no longer reproduce the flaky failure:

```console
$ go test -run=LoadChunk$ ./snapshots/ -count=1000 -failfast
ok      cosmossdk.io/store/snapshots    15.722s
```
---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed all author checklist items have been addressed
- [ ] confirmed that this PR does not change production code

store/snapshots/helpers_test.go

@@ -181,13 +181,24 @@ func setupBusyManager(t *testing.T) *snapshots.Manager {
 	mgr := snapshots.NewManager(store, opts, hung, nil, log.NewNopLogger())
 	require.Equal(t, opts.Interval, hung.snapshotInterval)
 
+	// Channel to ensure the test doesn't finish until the goroutine is done.
+	// Without this, there are intermittent test failures about
+	// the t.TempDir() cleanup failing due to the directory not being empty.
+	done := make(chan struct{})
+
 	go func() {
+		defer close(done)
 		_, err := mgr.Create(1)
 		require.NoError(t, err)
 		_, didPruneHeight := hung.prunedHeights[1]
 		require.True(t, didPruneHeight)
 	}()
 	time.Sleep(10 * time.Millisecond)
+
+	t.Cleanup(func() {
+		<-done
+	})
+
 	t.Cleanup(hung.Close)
 
 	return mgr