Add descriptions to the common assertions

Currently, when some of the assertions defined in `SpecHelpers.coffee` fail, the failure message can be rather ambiguous. Was:

```text
  1) test description (single subscriber) "before all" hook:

      Uncaught AssertionError: expected true to deeply equal false
      + expected - actual

      +false
      -true
```

Now:

```text
  1) test description (single subscriber) "before all" hook:

      no Initial events
      + expected - actual

      +false
      -true
```