Describe how test-mole can be used when library implements feature

Example from https://github.com/kensho/check-more-types -

``` js
(function registerAdditionalChecks(check) {
function defined(value) {
return typeof value !== 'undefined'; // 1
}
if (!check.defined) {
check.defined = defined;
}
}(window.check));
```

check-types.js now implements check.defined, we no longer test `// 1` code!
Test mole can actually test code even if it is not testable by the outside code.

Start with the check-types.js reference in the issue and the check-more-types example at the linked URL. Document how test-mole can exercise the defined function inside the closure, including the branch marked // 1; done when the usage is explained with a representative example.

Written by the indexing model from the issue text.