feat: is_slow test helper

As per the log:

```
    feat: is_slow test helper
    
    This patchset adds a test helper decorator that allows one to annotate
    tests as "slow". Slow tests will not be executed by a test runner
    unless the EOM_TEST_SLOW environment variable is nonempty.
    
    The *_limit tests for governor.py were decorated, as each of these was
    taking >30 seconds to run.
```