Add tests for KeyringController edge cases

[mikesposito]

Explanation

This PR adds additional tests for edge cases, useful for #3830.
Some istanbul ignore if statements are also being removed

References

• Fixes Bump KeyringController test coverage to 100% #3767
• See Merge EthKeyringController into KeyringController #3830

Changelog

No functional changes

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've highlighted breaking changes using the "BREAKING" category above as appropriate

[mcmire]

Looks good!

[mcmire]

Nit: Something I've started doing in other test files when it comes to withController (or other similar helpers, like setupController) is splitting off controller options from other kinds of options. So withController could take an options object, but the controller options could be bundled into an options object, and then any other setup configuration you want to make could be done in other options. Essentially that turns this type into:

type WithControllerOptions = {
  options: Partial<KeyringControllerOptions>;
  skipVaultCreation?: boolean;
}

I realize that this would require a change across the board, but something to think about perhaps for a future PR.

[mikesposito]

Good idea, I do think it would improve readability! I will do that in another PR