Currently, when a test class includes `AssertOffense`,
`AssertOffense#setup` attempts to dynamically identify the Cop under
test. If the Cop class cannot be found, `#setup` exits early.

In these cases, the test cases continue to execute with a nil `@cop`
since it wasn't set in `#setup` as expected. When helpers such as
`#assert_offense` are used, they access `@cop` and the tests can fail
with cryptic messages such as:

```
NoMethodError: undefined method `[]=' for nil
```

Initialization of the Cop was refactored to a lazily-called accessor
method which has the same caching functionality via `@cop`, but raises
an error if the expected Cop class isn't defined.

…sage-if-cop-not-defined

Tests using AssertOffense should Fail with a usable message if the relevant cop is not defined

…oops

They produce an assign node, but the value is part of the parent for node

The described problem doesn't occur anymore

Fix an error for `Minitest/SkipEnsure` when only `ensure` has a body

Fix an error for `Minitest/MultipleAssertions` when using for-style loops

Enable `InternalAffairs/UndefinedConfig`