Blocked by #5500 (fix is included in this PR so that CI will run).
Our unit test webpack config was erroneously sending in entry points to karma-webpack, who should receive no entry points.
This in turn was hiding errors related to typeRoots lookups.
It was also causing unit tests compilation to behave weirdly: unit test errors would not stop compilation, because other entries would still compile.
This might also have contributed to the overall slowness of unit tests in #5423.
Related to TypeStrong/ts-node#283Fix#3911Fix#5332Fix#5351