Cleanup testing & version checker usage.

[rwjblue]

VersionChecker usage changes:

• Ember is "singleton" (aka we are always using the top level version, and there can't me multiple nested ones), so swap to using VersionChecker(this) to grab the Ember version.
• Using the argument to included will not work for top-level addons (ones included in the project as direct deps or devDeps) because it has no .root property which is what ember-cli-version-checker uses. So change to this.parent when building up our "parent checker".

Testing infrastructure changes:

• Avoid mutating our own node_modules
• Use completely isolated testing root path for running tests
• Make testing harness reusable for other scenarios (future PR to add gte support)

[rwjblue]

Easier to review via https://github.com/pzuraq/ember-compatibility-helpers/pull/14/files?w=1.