Operating system counter stores signals as flag set without counter.
So sending two signals to a process may result to one or two signal
handler invocation. Queueing signals inside Idris could give users
false sense of signals being are queue, while they are not.
In particular, test for signal could not work reliably for that
reason.
Also, practically we usually don't need have more than once signal
event.
This is follow-up to #1660. CC @mattpolzin
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
