Until now, all unit tests were defined in Tests.hs. Pro: simple, makes
code/test line counting easy. Con: tests are far from code, Tests.hs turns
into a big wall of test code. Now, unit tests can also be defined in modules.
To avoid name clashes and template haskell complexity, a dumb manual
naming scheme is used: any module may export a hunit Test(List) named
tests_ModuleName. These are manually aggregated and re-exported when
appropriate, eg in Commands.All and finally in Tests.hs.
