Introduce a basic 'unittest' framework

[Geod24]

The intent is to progressively evolve to a pure unittest framework,
removing any filesystem dependency.
While the approach introduced here is not ideal, it makes tests
much easier to write, run, and provided the framework is ported
to Windows, opens up the possibility of running them on Windows.

Writting tests for dub is quite convoluted, so this is an attempt at simplifying the process. Feedback welcome.

[s-ludwig]

I didn't look through the individual tests, yet, but I'm in favor of this approach. We switched to a similar scheme internally some years ago, albeit without relying on fork, but rather by having a small shell script foreach over each test (calling the test binary without arguments lists all test identifiers, calling it with an argument will run the test matching the given identifier).

So AFAICS, the main open question is Windows support. It looks like we'd have to somehow compile in Posix mode for Cygwin to make the fork approach work? An alternative would be to let the main test invoke itself using std.process.execute and the command line argument approach. That way we would still stay in D land, without the need for a shell script loop.

[s-ludwig]

Why is this commented out?

[Geod24]

Debugging leftover, fixed

[Geod24]

I have a better alternative: #2759