refactor: remove test logic from lib

[erickzhao]

Cleans up our testing infrastructure.

Main change:

• Properly mock the electron module with jest.mock instead of passing a mock electron object as an undocumented parameter into the source code.

Other:

• Add ts-jest so that we're testing using TypeScript as well.
  • Remove trivial unit tests that now simply fail type assertions.
• Update Jest verbs to match what we use in our other repos (test -> it and toThrowError -> toThrow).
• Clean up package.json fixtures in tests.

[continuous-auth]

🎉 This PR is included in version 3.1.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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