Add tuple deformation test for virtual generated columns

Add tuple deformation test for virtual generated columns

Add coverage for a virtual generated NOT NULL column followed by a
physically stored NOT NULL column.  This exercises the tuple deformation
case fixed by 89eafad297a, where TupleDescFinalize() could incorrectly
treat a virtual generated column as part of the guaranteed physical column
prefix and compute cached offsets past it.

Without that fix, deforming the following column could read from the wrong
tuple offset.

Author: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/A4BC563C-0CA3-4EF3-952A-EA41F9E5BF1E%40gmail.com