CppEditor: Add test case for quickfix formatting

Also fixes an off-by-one error in the formatting code that was uncovered
by the test.

Change-Id: I013194e21cf37f318bb806bb60ea659b91b99fbf
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/cppeditor/cppquickfixes.h

@@ -402,6 +402,7 @@ public:
 
     enum class Mode {
             Off,         // Testing: simulates user canceling the dialog
+            Impl,        // Testing: simulates user choosing cpp file for every function
             Alternating, // Testing: simulates user choosing a different DefPos for every function
             User         // Normal interactive mode
     };