- Remove :group arguments from `defcustom` declarations
  - `defcustom` picks up `defgroup` names from the same file, so
    specifying the group is not necessary
- Ensure that all function-names are properly quoted using #'
- Add an explicit header-option setting lexical-binding to t
- Ensure that cl-check-type uses `satisfies` for filenames
- Replace all `defadvice` with `advice-add`
- Replace `(list ...)` patterns with `\`(...)` patterns

Fixes: #62
Fixes: #32
Partially Fixes: #24

"test-helper.el" isn't a library intended to be loaded with `require`.
Instead it is loaded with `load` by `ert-runner` itself.  Because of
that, it is confusing to provide a feature using `provide`.  It also
isn't possible to use `require' to load `test-helper` because that
might, depending on the order of `load-path` load the "test-helper.el"
of another package that used `ert-runner`.

This is also discussed at rejeep/ert-runner.el#38.