serializable installcheck

I've discovered (the hard way) that if you run `make installcheck`
with default_transaction_isolation = 'serializable' and you have a
serializable read write transaction sitting idle (or prepared) that
the "transactions" test will block indefinitely waiting for a safe
time to run, because of the test of the READ ONLY DEFERRABLE code.
I think that's OK, but figured I should make people aware of the
fact and see if everyone agrees.

I will admit that it can be confusing until you find the idle or
prepared transaction.

-Kevin