Add automated tests

[ehmicky]

Fixes #18.

This adds some automated integration tests.

The test uses the already existing test fixture integration-test and runs it with @netlify/build. Then:

• It checks that the manifest.json exists and is valid
• It loads the bundled file and verify that netlifyRegistry.set() is correctly called with each handler
• It calls each handler's onRequest() and verify they are executing correctly

The tests are performed by npm test, which is automatically run in CI (GitHub actions).
nyc is used for test coverage.

When running the tests with the environment variable DEBUG=1, the build logs from @netlify/build are printed, which allows using the tests in debugging.

[ehmicky]

This is ready for review!
The tests are failing on Windows, but I created a separate issue at #36 for it, since it seems to be a bug in the source.

[shortdiv]

Looks good, tests are passing!