Cutting test runtime for src/test/modules/snapshot_too_old

I've complained before that the snapshot_too_old TAP tests seem
ridiculously slow --- close to a minute of runtime even on very fast
machines. Today I happened to look closer and realized that there's
an absolutely trivial way to cut that. The core of the slow runtime
is that there's a "pg_sleep(6)" in the test case; which perhaps could
be trimmed, but I'm not on about that right now. What I'm on about
is that two of the three isolation tests allow the isolationtester to
default to running every possible permutation of steps, one of which
doesn't even generate the "snapshot too old" failure. IMV it's
sufficient to run just one permutation. That opinion was shared by
whoever wrote sto_using_hash_index.spec, but they didn't propagate
the idea into the other two tests.

The attached cuts the test runtime (exclusive of setup) from
approximately 30+24+6 seconds to 6+6+6 seconds, and I don't see
that it loses us one iota of coverage.

I cleaned up some unused tables and bad comment grammar, too.

regards, tom lane