alter-table-1 isolation test spec contains a duplicate step name

Commit f14a6bbedb79adce2298d0d4f5e2abe8563e0eca added an isolation
test, alter-table-1.spec, which reads in part:

session "s2"
setup           { BEGIN; }
step "rx1"      { SELECT * FROM b WHERE a_id = 1 LIMIT 1; }
step "wx"       { INSERT INTO b VALUES (0); }
step "rx1"      { SELECT * FROM b WHERE a_id = 3 LIMIT 3; }
step "c2"       { COMMIT; }

The step name "rx1" is used twice, a condition the isolation tester
should probably reject as an error but currently doesn't.  This
happens to work OK at the moment because we don't specify a list of
permutations, and except for that, the names aren't used for anything
that cares about whether they are duplicated.  Nonetheless, this seems
like it should probably be changed.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company