Select the bitmap out of wxDC before using it in the unit test

The unit test added in 607b800444 had a bug as
it tried to use the bitmap directly while it was still selected into a
wxMemoryDC, which wasn't guaranteed to work and resulted in an assert.

Fix this by destroying wxMemoryDC earlier.

See #17666.

tests/graphics/bitmap.cpp

@@ -93,7 +93,7 @@ void BitmapTestCase::OverlappingBlit()
     m_bmp.SetMask( NULL );
 
     // Clear to white.
-
+    {
     wxMemoryDC dc(m_bmp);
     dc.SetBackground( *wxWHITE );
     dc.Clear();
@@ -106,6 +106,7 @@ void BitmapTestCase::OverlappingBlit()
     // Scroll down one line.
 
     dc.Blit( 0, 1, 10, 9, &dc, 0, 0 );
+    } // Select the bitmap out of the memory DC before using it directly.
 
     // Now, lines 0 and 1 should be red, lines 2++ should still be white.