Introduce a way to easily override current time in tests

[ma2ciek]

Testing error handling depending on time is pretty complex because sinon.useFakeTimers() make async functions synchronous, which makes promises crashing instantly.

E.g. the following test will fail instantly when the CKEditorError error is thrown:

const watchdog = Watchdog.for( ClassicTestEditor );
const originalErrorHandler = window.onerror;
const clock = sinon.useFakeTimers();

window.onerror = undefined;

return watchdog.create( element ).then( () => {
	setTimeout( () => throwCKEditorError( 'foo', watchdog.editor ), 1000 );

	return new Promise( res => {
		watchdog.on( 'restart', () => {
			window.onerror = originalErrorHandler;

			watchdog.destroy().then( res );
		} );

		clock.tick( 1000 );
	} );
} );

as it boils down to something like this:

// some code
return new Promise( res => {
	// Some code.
	throwCKEditorError( 'foo', watchdog.editor );

	// Some code.
	res();
} );

Code after the error isn't executed, thus the test fails instantly. If the error would be in the setTimeout, even with timeout 0, the rest of the code would execute and such error wouldn't have an impact on the test behavior.

Thus I'm proposing adding the protected _now() method to the watchdog, which would return the Date.now(), so it could be overridden later in tests without affecting other mechanisms depending on time.