Wait for all goroutines to finish
Wait for all goroutines to finish

Initially, we did not wait for all goroutines to finish which was fine
as long as we did not run all tests with `go test` and observe deadlock
in 50% of cases. This is interesting, since deadlock never occurred when
running safe_prime_generator_test.go. Anyway, waiting for all go
routines to finish shouldn't harm the performance a lot and it gives a
guarantee we did not miss anything.

didiercrunch/paillier