[bug] multiple before() or after() empty blocks can be added to a test suite

[ngarbezza]

Describe the bug
before() and after() are designed so that there can be only one before and after in a test suite, and it needs to have a function as an argument. Passing no parameters should be considered invalid, as well as having more than one declaration.

Version(s) affected
v5.1.0

To Reproduce

Run a test suite with the following content:

const { suite, before } = require('@pmoo/testy');

suite('wrong!', () => {
  before();
  before();
});

Expected behavior
Raise an error because of two reasons:

• before() should receive a function (reported here: [bug] before() and after() argument is not validated #227)
• more than one before() per suite is not allowed (there's a validation in place but it's not catching this particular case)