Use wxAlphaPixelData for default native bitmaps in the unit test

The unit test added in 607b800444 didn't work on
the systems using 32 bpp screens, i.e. almost all of them, because it tried to
use 24 bpp wxNativePixelData with them.

Fix this by using wxAlphaPixelData, which won't work on non 32 bpp systems,
and so is still not ideal, but at least makes the test work on most systems by
default.

See #17666.

tests/graphics/bitmap.cpp

@@ -110,8 +110,8 @@ void BitmapTestCase::OverlappingBlit()
 
     // Now, lines 0 and 1 should be red, lines 2++ should still be white.
 
-    wxNativePixelData npd( m_bmp );
-    wxNativePixelData::Iterator it( npd );
+    wxAlphaPixelData npd( m_bmp );
+    wxAlphaPixelData::Iterator it( npd );
 
     ASSERT_EQUAL_RGB( it, 255, 0, 0 );
     it.OffsetY( npd, 1 );