Refactor tests

- Migrate benchmarks and regression tests to tap,
- Replace the "bench" make target by "short_bench" and "long_bench".
  The short benchmark is 3 seconds per test, and the long one is 30
  seconds per test,
- make regtest now invokes the benchmarks with only 1 second per
  benchmark.
- Now use "nproc" command to detect the number of available CPUs rather
  than hardcoding a value.
- rcutorture in "stress" mode is now executed.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>