Join all threads created in tests
Join all threads created in tests

A lot of tests created threads but never waited (joined) on them. As a
result if one of those threads panicked they test still passed and the
test runner exits with a success exit code.

Updates #321.

aturon/crossbeam,crossbeam-rs/crossbeam,crossbeam-rs/crossbeam,crossbeam-rs/crossbeam,crossbeam-rs/crossbeam