Initial test coverage for viewport selection before introducing new 'single' select functionality (#3987)

* WIP changes for viewport single select and better integration tests

Signed-off-by: hultonha <hultonha@amazon.co.uk>

* update test to verify single click selection in the viewport

Signed-off-by: hultonha <hultonha@amazon.co.uk>

* temporarily remove changes for single select

Signed-off-by: hultonha <hultonha@amazon.co.uk>

* temporarily remove ChangeSelectedEntity call

Signed-off-by: hultonha <hultonha@amazon.co.uk>

* initial tests to validate current viewport selection model

Signed-off-by: hultonha <hultonha@amazon.co.uk>

* some tidy-up before publishing PR

Signed-off-by: hultonha <hultonha@amazon.co.uk>

* add reference to const type in loop

Signed-off-by: hultonha <hultonha@amazon.co.uk>

* temporarily disable box select test

Signed-off-by: hultonha <hultonha@amazon.co.uk>

Code/Framework/AzManipulatorTestFramework/Source/IndirectManipulatorViewportInteraction.cpp

@@ -40,6 +40,12 @@ namespace AzManipulatorTestFramework
     {
         m_viewportInteraction.UpdateVisibility();
 
+        // ensure we call display viewport 2d to simulate this update step (some state may be
+        // updated here, e.g. box select)
+        AzFramework::ViewportDebugDisplayEventBus::Event(
+            AzToolsFramework::GetEntityContextId(), &AzFramework::ViewportDebugDisplayEvents::DisplayViewport2d,
+            AzFramework::ViewportInfo{ m_viewportInteraction.GetViewportId() }, m_viewportInteraction.GetDebugDisplay());
+
         DrawManipulators();
         AzToolsFramework::EditorInteractionSystemViewportSelectionRequestBus::Event(
             AzToolsFramework::GetEntityContextId(),