Improve generated_stored test

Improve generated_stored test

The test table names gtest11s and gtest12s were way originally chosen
to signify "stored", when the idea was to have virtual columns in the
same test file.  This is no longer the idea, so this naming is
irrelevant.  (The upcoming feature of virtual generated columns will
have a test file that is initially a copy of generated_stored.sql, and
this random difference will be even more annoying then.)  Clean this
up by dropping the suffix.

Discussion: https://www.postgresql.org/message-id/flat/a368248e-69e4-40be-9c07-6c3b5880b0a6@eisentraut.org