Add test coverage scripts to package.json for code coverage workflow

[fridgepoet]

What this PR does / why we need it:
Adds the expected frontend test coverage script for the code coverage workflow.

Which issue(s) this PR fixes:

Contributes to #153

[github-actions]

Backend code coverage report for PR #162
No changes

[github-actions]

Frontend code coverage report for PR #162
No changes

[github-actions]

Levitate is-compatible report:

🔍 Resolving @grafana/data@latest...
🔍 Resolving @grafana/ui@latest...
🔍 Resolving @grafana/runtime@latest...
🔍 Resolving @grafana/e2e-selectors@latest...

🔬 Checking compatibility between ./src/module.ts and @grafana/data@9.3.6...
✔ Found @grafana/data version 8.4.11 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/ui@9.3.6...
✔ Found @grafana/ui version 8.4.11 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/runtime@9.3.6...
✔ Found @grafana/runtime version 8.4.11 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/e2e-selectors@9.3.6...
Skipping package @grafana/e2e-selectors because it is not used in the project or not installed locally.
did you forget to run yarn install or npm install?

✔️ ./src/module.ts appears to be compatible with @grafana/data,@grafana/ui,@grafana/runtime,@grafana/e2e-selectors