refactor(kiln test): encapsulate containerized tile testing

[crhntr]

This change introduces a new private package called "test" (full name "github.com/pivotal-cf/kiln/internal/test"). It exports two identifiers. test.Run and test.Configuration. The former is a function with signature:

func Run(ctx context.Context, w io.Writer, configuration Configuration) error { return nil }

The Configuration structure is defined as:

type Configuration struct {
	SSHSocketAddress string

	AbsoluteTileDirectory string

	RunAll,
	RunMigrations,
	RunManifest,
	RunMetadata bool

	GinkgoFlags string
	Environment []string
}

All the internal docker/ssh-agent configuration is unit tested via package internal tests so that the public interface used in the commands package is easier to test and understand.

Fixes

This fixes an issue where all kiln commands will fail if the ssh-agent is not properly configured. Now it will only fail on kiln test.

This makes the command tests run much faster and makes command tests run on the order of seconds not minutes. By default the integration test between the ./internal/test package and Docker is run; however, for fast feedback a kiln developer can run test with the -short flag. On my machine I can run go test -short ./... from the project root and it finishes in a couple seconds.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. Unfortunately, the Pivotal Tracker project is private so you may be unable to view the contents of the story.

The labels on this github issue will be updated when the story is started.

[notrepo05]

I'd suggest a failure case due to the high impact on CI if we fail to return the error properly. Or fix this if there's an issue in implementation. Looks great though!