Coerce args.functionalSuites to array

Fixes #272.
Rename `received` property due to upstream change

Fixes #265.
Allow empty suites/functionalSuites command-line args

Also ensure that the state of `args.suites` is used when determining
whether or not to run ClientSuite in the runner, since that argument
is passed to the client and decides which unit test suites execute.

Fixes #268. Closes #269.
Empty suites command-line flag should skip running client

Refs #241. Closes #270.
Update Leadfoot dependency
Update contribution guidelines

[ci skip]
Update contribution guidelines

Let's be extra clear.

[ci skip]
Use less stupid code to perform suites overrides. Thanks @Blasz

Refs #269.