Today this code will crash the shell with what appears to be undefined behavior

load('jstests/libs/parallelTester.js');
var a = fork(function(a, b) {
    return a / b;
}, 10, 2);
a.start();

If you run the snippet in gdb, it will show you that we're calling terminate() because the std::thread was never joined. We can provided an easier-to-debug experience for developers by tracking thread creation ourselves, and checking for open threads (and their source location) on shutdown of the test runner.