Support for "Simulator device set"

[RobinDaugherty]

This is an attempt to address #24.

Mussel client infers the device set based on the HOME environment variable, which contains a path to the application within the Simulator. If it contains the string "XCTestDevices", then we use the device set name "testing". However, this could be changed to specify the path of the device set instead of the magic name "testing" by truncating it.

I changed the client to use a base class instead of protocol so that shared behavior can be written once. Identifying the current simulator ID and device set are in the base class. The base class implements the serverRequest(task:options:) method, and layers in these two items.

On the server, if a request includes the new option, it's used in the simctl command line. I also changed this to fail early if the weak self has gone away, rather than return a success status.

[ridvank]

This is what we are missing too unfortunately, is this repo still active? If yes, it would be amazing to merge this PR