Standardise entry points for Complement tests

[kegsay]

Every test package that wants to use Complement needs some bootstrapping boilerplate code to do things like read the env vars and clean up old containers in case of an ungraceful shutdown. There is a convenient TestMain function which is part of standard Go tests which we can use for this.

We also had a helper function called Deploy which would deploy a set of containers and return the docker.Deployment for tests to use. This helper function is actually boilerplate as well, but has still historically been defined for each test package unnecessarily.

This PR cleans up these entry points in the following way:

• Always use the helper functions when writing entry points. Previously, only MSC packages did this. This was just an omission from Remove msc build tags. Add msc packages. #666
• Moves entry point functions from the helpers package to the top-level complement package. Why: this signals to test writers where to look when they want to hook into Complement machinery as it serves as the entry point functions. This ends up reading very nicely as deployment := complement.Deploy(t, b.BlueprintAlice).
• Remove the useless proxy function Deploy(...) which just called the previous line.

This now means that out-of-repo tests (which need to call somepackage.Deploy) and in-repo tests look identical, and bootstrapping boilerplate is as small as possible:

package tests

import (
	"testing"

	"github.com/matrix-org/complement"
)

func TestMain(m *testing.M) {
	complement.TestMain(m, "package-name-goes-here")
}

[DMRobertson]

Thanks for the clean commit history. Seems sane to me.