time::since_epoch test crashes with overflow when debug assertions are enabled

On Mac OS, I get the following when building with `--enable-debug-assertions`:

```
---- time::tests::since_epoch stdout ----
thread 'time::tests::since_epoch' panicked at 'arithmetic operation overflowed', src/libstd/sys/unix/time.rs:91
```

We should track down what's going on with this test, and get the builders enabling debug assertions in general to catch this kind of problem.
