This fixes problem of creation the garbage of zombie processes on POSIX
systems. This also makes behaviour of `popen2` identical in Windows,
namely, all resources are freed only when waiting, giving at the same
time an ability to observe the exit code afterwards.
While the discussion about how to refactor test framework is not
finished (#1654), make this change: move `rm -rf build` in the
beginning of the test. For these reasons:
* it is useful to inspect the contents of the `build` directory
especially after the test failure
* if build crashes mid-test (e.g. process killed), next run should
not be affected by the `build` directory from the previous run
