Previously, all log messages generated by tox_new (which is quite a lot)
were dropped, because client code had no chance to register a logging
callback, yet. This change allows setting the log callback from the
beginning and removes the ability to unset it.
Since the log callback is forever special, since it can't be stateless,
we don't necessarily need to treat it uniformly (with `event`).
Compiling as C++ changes nothing semantically, but ensures that we don't
break C++ compatibility while also retaining C compatibility.
C++ compatibility is useful for tooling and additional diagnostics and
analyses.
In a next step, we will remove tests from each file to have a per-binary
split of tests. This will help identify which tests fail most often on
Travis CI.
In another future step, we will split the large one_test into several
auto tests, which will make testing quite a bit slower (adding about 10
seconds setup time to each), but hopefully a lot more stable ("Tox went
offline" should not happen as much anymore).
