Don't change orthogonal alignment in wxSizerFlags::CenterXXX()

It was unexpected that using wxSizerFlags().Right().CentreVertical()
didn't right-align the item because CentreVertical() reset the alignment
in the horizontal direction rather than just setting it in the vertical
one, so change this, document the new behaviour explicitly and add a new
unit test checking for it.

Closes #18989.

interface/wx/sizer.h

@@ -1455,6 +1455,9 @@ public:
         them anyhow. For 2D sizers, centering an item in one direction is quite
         different from centering it in both directions however.
 
+        Note that, unlike Align(), this method doesn't change the vertical
+        alignment.
+
         @see CentreVertical()
 
         @since 3.1.0
@@ -1467,6 +1470,9 @@ public:
         The remarks in CentreHorizontal() documentation also apply to this
         function.
 
+        Note that, unlike Align(), this method doesn't change the horizontal
+        alignment.
+
         @since 3.1.0
      */
     wxSizerFlags& CentreVertical();