Allow context injection in core-block tests

[mirka]

Description

This adds the ability to inject context when testing core-block components.

This will allow components using a context-dependent HOC (like withAPIData) to be snapshot tested using the same blockEditRender() test helper.

Example of intended usage

For a quick test, you can check out this commit and try the following as a test case in core-blocks/file/test/index.js

test( 'block edit matches snapshot', () => {
	const contextTypes = {
		getAPISchema: noop,
		getAPIPostTypeRestBaseMapping: noop,
		getAPITaxonomyRestBaseMapping: noop,
	};

	const context = Object.assign( {}, contextTypes );
	context.getAPISchema = () => {
		return { routes: null };
	};

	const wrapper = blockEditRender( name, settings, context, contextTypes );

	expect( wrapper ).toMatchSnapshot();
} );

[gziolo]

I just wanted to raise one issue with this proposal. Please note that it is using Legacy Context which is going to be deprecated in React 17. In my opinion, we shouldn't be investing time in extending our test in code that won't work in the future. We should rather to seek an alternative approach for File block to avoid using withAPIData which we are slowly replacing with wp.data.withSelect.

[gziolo]

I came to the conclusion that we should close this one after leaving my comment on #6805:

You should be rather using getMedia selector instead of withAPIData which is going to be deprecated as soon as we find a way to port all existing code that handles REST API requests. This will also remove the need to introducing changes proposed in #7108.