autop: Include tests for removep functionality

[aduth]

Previously: #15128 (comment)

As described at #15128 (comment), due to the method of porting and adapting autop and removep functions from previous core equivalent PHP and JavaScript implementations, there were no unit tests included for removep.

For historical context, the module originated as a direct port of the PHP implementation of autop, including its tests. There is no removep in PHP, so there are no tests. There was a previous removep implementation in JavaScript from which this implementation was adapted, but it had no tests to port over.

This function is notable due to its potential impact on content produced for posts consisting of legacy content (only containing the Classic block).

gutenberg/packages/editor/src/store/selectors.js

Lines 821 to 831 in d5bdd53

	// For compatibility purposes, treat a post consisting of a single
	// freeform block as legacy content and downgrade to a pre-block-editor
	// removep'd content format.
	const isSingleFreeformBlock = (
	blocks.length === 1 &&
	blocks[ 0 ].name === getFreeformContentHandlerName()
	);
	if ( isSingleFreeformBlock ) {
	return removep( content );
	}

Task: Add unit test coverage for the removep function.

[gziolo]

Let's move it to the tracking issue that lists all public API functions in packages that don't contain unit tests and link back to this one.