Ability to run tests in watch mode

[Tommy]

The current test suite takes a while to startup and run - in order, it:

1. Deletes the dist directory
2. Builds the source and test files and outputs to the dist directory
3. Copies the test fixtures to the dist directory
4. Runs ESLint
5. Runs Ava

With steps 1-4, it can take 5-10 seconds before any test even runs! When working on a feature, it would be great to be able to run a set of tests in watch mode, so you could have quick feedback on the changes you're making. Currently, that's hard to do, as parts of steps 2 and 3 can't be run in watch mode.

Some potential solutions:

• Update cpy-cli to add a watch mode
  • see sindresorhus/cpy-cli#16, which says the feature must first be added in sindresorhus/cpy
  • cpy has a blocker on sindresorhus/cpy#99
• Use a bundler for building/testing
  • Something like Rollup would make building/copying/watching (and in the right order) fairly straightforward
• Restructure the fixtures directory to allow symlinking to the dist directory
• Use something like ts-node to test TypeScript files directly, without a build step

I'd be open to making a PR for any of these, but I'm not familiar with the issue in cpy to add test cases for it. Personally, I'm fairly familiar with Rollup.

[Tommy]

Relevant scripts:

https://github.com/SamVerschueren/tsd/blob/621aad986c636ff511293706455720b006598945/package.json#L18-L26