Improve test readability of loadContext and pluginLoaders

[Beanow]

There was a fair amount of copy-pasted lines in these tests. Which is mostly a
good thing, because ESlint and Flow provide good errors when you're using the
variables wrong. But in terms of readability wasn't great.

In upcoming PRs we'll add more to these test files. So I thought it was good to
improve this first.

This version:

• Still copy-pastes to get good ESlint and Flow errors.
• Doesn't repeat itself when it's not for better errors or readability.
• Uses a "spyBuilder", for readability in spite of prettier trying to collapse lines.
• Makes sure duplicates are exactly duplicates, easier to edit in IDEs.
  example: [githubDeclaration, "githubDeclaration"]
  instead of [fakeGithubDec, "fake-github-dec"]

Test plan: yarn test

Reviewer note: recommend looking at the split diff on Github, not the unified one.

[teamdandelion]

Thanks! I really appreciate cleanup PRs like this. They make me feel good about our codebase. 💜