Add alter_generic regression test

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: ff7e5b4841eeedfb9cefec6226f4dca5b8840276
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2012-09-28T21:39:12Z
Releases: 9.3.0
Add alter_generic regression test

This makes refactoring of parts of the ALTER command safe(r) because we
ensure no change in functionality.

Author: KaiGai Kohei

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