[Infra UI] Fix flyout-related tests (#163469)

closes [#162672](#162672)
## Summary

This PR fixes the `should render alerts count for a host inside a
flyout`. It also moves all flyout-related tests to the `#Single Host
Flyout` test.


### How to test

```bash
yarn test:ftr:server --config x-pack/test/functional/apps/infra/config.ts
```

```bash
yarn test:ftr:runner --config x-pack/test/functional/apps/infra/config.ts --include x-pack/test/functional/apps/infra/hosts_view.ts
```


https://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/2830