refactor: Extract common functionality to test utils pkg

[salvacorts]

What this PR does / why we need it:
This PR extracts some of the functions we use for testing in the planner pkg into a new test_utils pkg so we can reuse them to test the planning strategies w/o duplicating code.

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[rfratto]

extremely tiny nit: Generally I prefer package names being slightly more descriptive about their domain than utils/testutils.

plannerfakes, plannerutils, or testplanner would all be slightly more descriptive here, in the same vein as the stdlib slogtest package.

That being said, of my opinions about Go, I feel the least strongly about this one, so I don't mind if this stays the same, either.

[salvacorts]

Done, renamed to plannertest. Thank you for the tip!