Tests: add manual test (widget - keystroke).

tests/plugins/widget/manual/keystroke.html

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+<div id="editor">
+	Hello world.
+</div>
+
+<script>
+	CKEDITOR.replace( 'editor', {
+		keystrokes: [
+			[ CKEDITOR.CTRL + 13 /*Enter*/, 'source' ]
+		]
+	} );
+</script>

tests/plugins/widget/manual/keystroke.md

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+@bender-tags: widget, bug, 4.14.1, 3998
+@bender-ui: collapsed
+@bender-ckeditor-plugins: toolbar, wysiwygarea, sourcearea, widget
+
+1. Open console in dev tools.
+
+1. Place cursor inside editor.
+
+1. Press <kbd>CTRL</kbd> and <kbd>Enter</kbd> key.
+
+	### Expected
+
+	Source mode is opened and no error appears in dev console.
+
+	### Unexpected
+
+	Error appears in dev console or source mode editor is not opened.
+
+1. Place cursor inside editor.
+
+1. Press <kbd>CTRL</kbd> and <kbd>Enter</kbd> key.
+
+	### Expected
+
+	Editor switches to WYSIWYG mode. No error appears in dev console.
+
+	### Unexpected
+
+	Error appears in dev console or WYSIWYG area is not opened.