Re-enable *mandatory* fastboot tests in CI

[mike-north]

The fastboot CI job has been failing on master for about a month, probably due to a combination of

• ember-data#master being incompatible with recent releases of ember (you'll get a rollup build error when attempting to build)
• ember-fetch's custom rollup stuff not working nicely with fastboot testing infrastructure

To address this, I've improved our CI configuration (#1718) to detect this kind of breakage in nightly builds, by ensuring that tests are run both with and without taking the lockfile into account.

Unfortunately, getting fastboot testing passing again will take some non-trivial digging. Here's what I've found in my limited time

• in https://github.com/kaliber5/ember-fastboot-addon-tests most dependency versions are "borrowed" from the library being tested. ember-data is an exception, which defaults to ember-data#master. I've opened a PR here which at least gets the first test running again [Feature] --ember-data-version option kaliber5/ember-fastboot-addon-tests#26
• ember-fetch's custom rollup stuff seems to be failing when it fails to find (I think) babel-preset-env in the fastboot test app. I haven't had a chance to dig into this any further yet

In the mean time, to allow meaningful development to continue while some of this infrastructure is updated/repaired, I've marked the CI job as an allowed failure.

[mike-north]

Good news! the fastboot testing job stopped hanging as of this commit dc6be17.

We may be good to put this job back on the critical path again