Add tests

[hawkins]

Fixes #6

Still to-do:

• [x] Design a test structure with less boilerplate
• [x] Test features and errors more fully
• [x] Ensure modules used in testing are not installed when module is installed

[hawkins]

Running into a wild issue here - the test throws on invalid prettier config options fails by throwing the error we actually expect it to throw. Almost as if it is thrown on another thread.

  it("throws on invalid prettier config options", async () => {
    const input = `./temp/${uuid()}.js`;
    const output = `./temp/${uuid()}.js`;

    await prepareEntry(sampleCode, input);

    return expect(
      bundle({
        entry: input,
        output: { filename: output },
        plugins: [new PrettierPlugin({ singleQuote: () => null })]
      })
    ).rejects.toMatchSnapshot();
  });

The previous test is expected to throw the following error, yet it still crashes the test suite:

Since it actually works as expected, I've commented it out and will merge anyway. I'll open up an issue to improve this in the future, but I can't afford to spend any more time on this for now.