Figure out why we need build prior to running tests

[Benoît Lemaire]

It seems like build (tsc) must be done prior to running tests. This shouldn't be necessary as the code is transpiled on the fly using ts-register. I also noticed some coverage inconsistencies where I had to build prior to running coverage (which also run a build).
Should get to the bottom of this. Ideally we shouldn't have to build prior to running the test suite.