test: add more panel tests

[Proximyst]

As it says on the tin, really. Only test files are touched.

[github-actions]

🐳 Docker image built and pushed to GitHub Container Registry.

You can pull it using:

# For the Node.js server:
docker pull ghcr.io/grafana/grafana-image-renderer:dev-pull-794-578da3df3af734d0c02392a8225f59dd0d9a2a30
# For the Go server:
docker pull ghcr.io/grafana/grafana-image-renderer:dev-pull-794-578da3df3af734d0c02392a8225f59dd0d9a2a30-golang

Warning

This is a development image and should not be used in production.
It will be automatically removed after 2 weeks.

[github-actions]

😢 zizmor failed with exit code 14.

Expand for full output

error[cache-poisoning]: runtime artifacts potentially vulnerable to a cache poisoning attack
  --> ./.github/workflows/build-plugin.yaml:36:9
   |
 3 | / on:
 4 | |   push:
 5 | |     branches:
 6 | |       - master
 7 | |     tags:
 8 | |       - 'v*'
 9 | |   pull_request:
   | |_______________- generally used when publishing artifacts generated at runtime
...
36 |         - uses: actions/setup-node@v4
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^ cache enabled by default here
   |
   = note: audit confidence → Low

16 findings (2 ignored, 13 suppressed): 0 informational, 0 low, 0 medium, 1 high