Add tests for ResultsPanel selectors (100% coverage for all selectors)

[schoinh]

Problem

Resolves #182

This is the last PR for the above issue! After this is merged, all selectors in simularium-website will be 100% covered.

Solution

Added tests to the ResultsPanel container's selector and its helper functions, breaking down the functions further to make them more easily testable on the way.

Type of change

Please delete options that are not relevant.

• This change requires updated or new tests

Steps to Verify:

1. Check that refactoring the selector and helper functions didn't break the plots. Run npm start, load some models with plots (like endocytosis) and make sure the plots look formatted the same way as before.
2. GitHub Actions should take care of test coverage check

Keyfiles (delete if not relevant):

1. src/containers/ResultsPanel/selectors.test.ts - All new tests
2. src/containers/ResultsPanel/selectors.ts - Broke out configureLayout helper function into smaller functions to make them more easily testable; turned some style values into constants

[github-actions]

jest coverage report 🧪

Total coverage

Status	Category	Percentage	Covered / Total
🟡	Statements	75.51% (+1.28% 🔼)	518/686
🟡	Branches	70.95% (+1.23% 🔼)	105/148
🔴	Functions	43.75% (+3.03% 🔼)	77/176
🟡	Lines	75.64% (+1.32% 🔼)	500/661

Status of coverage: 🟢 - ok, 🟡 - slightly more than threshold, 🔴 - under the threshold

Show new covered files 🌑

Coverage of new files

Status	Filename	Statements	Branches	Functions	Lines
🟢	src/containers/ResultsPanel	100%	100%	100%	100%

Status of coverage: 🟢 - ok, 🟡 - slightly more than threshold, 🔴 - under the threshold

[schoinh]

This is duplicate logic so I removed it. TypeScript wouldn't allow a null or undefined or other falsy value to be passed in as plotData. And if plotData were an empty array, the map function below would just return an empty array anyway.