TestTableAlterModifyColumn: cover new col reorder case [ci skip]
TestTableAlterModifyColumn: cover new col reorder case [ci skip]

As described at the bottom of https://github.com/skeema/skeema/issues/36, there
are situations in which Table.Diff() is attempting to reposition the same col
multiple times, generating illegal SQL. This commit adds test coverage for this
scenario. The test currently fails since no fix has been committed yet.

skeema/tengo,skeema/skeema