Run tests in DEBUG in CI (in addition to RELEASE)

[roji]

Our CI runs tests in RELEASE, since that's what we release to users. However, we have various additional DEBUG-only checks and asserts in the codebase, and it's important these get evaluated in CI as well.

We can add an additional build for DEBUG, or maybe leave Helix in Release and make the non-Helix build run in DEBUG?

[ajcvickers]

Note from triage: developers frequently run the tests in Debug mode which should catch any issues relatively quickly, and we don't want to either lose any platform/environment coverage for testing what we actually ship, or add new runs since the C.I. system is already painfully slow.