Restore lost amcheck TOAST test coverage.

 Restore lost amcheck TOAST test coverage.
 
 Commit eba77534 fixed an amcheck false positive bug involving
 inconsistencies in TOAST input state between table and index.  A test
 case was added that verified that such an inconsistency didn't result in
 a spurious corruption related error.
 
 Test coverage from the test was accidentally lost by commit 501e41dd,
 which propagated ALTER TABLE ...  SET STORAGE attstorage state to
 indexes.  This broke the test because the test specifically relied on
 attstorage not being propagated.  This artificially forced there to be
 index tuples whose datums were equivalent to the datums in the heap
 without the datums actually being bitwise equal.
 
 Fix this by updating pg_attribute directly instead.  Commit 501e41dd
 made similar changes to a test_decoding TOAST-related test case which
 made the same assumption, but overlooked the amcheck test case.
 
 Backpatch: 11-, just like commit eba77534 (and commit 501e41dd).