Add a functional test suite

[kumar303]

There's no avoiding it, we need a functional test suite. There is just no way to cover all end user functionality with our unit test suite. The latest regression had to do with missing yargs configuration. There have been a few others like this.

Goals:

• test only things that cannot be covered in unit tests
• make them as fast as possible
• they cannot be flaky

[kumar303]

another regression example: #380 (a call that failed without mock injection)

[kumar303]

added in #469 !