function onFinish( clbk ) {
	var i;
	if ( !isFunction( clbk ) ) {
		throw new TypeError( format( 'invalid argument. Must provide a function. Value: `%s`.', clbk ) );
	}
	// Allow adding a listener only once...
	for ( i = 0; i < listeners.length; i++ ) {
		if ( clbk === listeners[ i ] ) {
			throw new Error( 'invalid argument. Attempted to add duplicate listener.' );
		}
	}
	listeners.push( clbk );
}
Adds a listener for when a harness finishes running all benchmarks.

@private
@param {Callback} clbk - listener
@throws {TypeError} must provide a function
@throws {Error} must provide a listener only once
@returns {void}