Get CI passing

[dfreeman]

We had two things keeping CI from passing:

• A bad interaction between ember-source and some Babel tooling ([BUGFIX lts] Fix transpilation issues (e.g. _createSuper is not a function) when used with Babel 7.9.0+. emberjs/ember.js#18831)
• The introduction of Report primitive type in literal-to-primitive relation complaints microsoft/TypeScript#38049, which just changed the output we were expecting slightly in one test.

For the former, I bumped ember-cli-babel and ember-source, and for the latter I changed the type error we introduced to something that would produce a consistent error message across our supported TS versions.