Add validity middleware mock and tests

[nicknisi]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:

This adds a validity middleware mock and tests for it.

const App = factory(({ middleware: { validity } }) => {
	const { valid, message } = validity.get('test', 'test value');
	return !valid ? <div>{message}</div> : <div>valid</div>;
});

validityMock('test', { valid: false, message: 'test message' });
let h = harness(() => <App />, { middleware: [[validity, validityMock]] });
h.expect(() => <div>test message</div>);

Resolves #630

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[matt-gadd]

is this meant to be here?

[nicknisi]

Oops, no I meant to remove that. I originally added it because value is unused and I was quieting TypeScript. Renaming the argument to _value to quiet TypeScript instead.