Remove registerTestPlatform()

[nex3]

Now that the Loader is the canonical place for determining which
TestPlatforms are valid, we no longer need a separate mechanism for
registering platforms. Adding a plugin to the loader is sufficient.

See #391

[grouma]

I fail to see how we will register a new platform in the loader. Can you give a small code snippet?

[nex3]

loader.registerPlatformPlugin([
  new TestPlatform("name", "identifier")
], () => new MyPlatformPlugin());

Or, using the top-level hack API:

registerPlatformPlugin([
  new TestPlatform("name", "identifier")
], () => new MyPlatformPlugin());

[grouma]

Ahh it's clear to me now. Thanks! LGTM