Regression tests vs SERIALIZABLE

Hi,

While reviewing the patch for parallel REFRESH MATERIALIZED VIEW, I
noticed that select_parallel.sql and write_parallel.sql believe that
(1) the tests are supposed to work with serializable as a default
isolation level, and (2) parallelism would be inhibited by that, so
they'd better use something else explicitly. Here's a patch to update
that second thing in light of commit bb16aba5. I don't think it
matters enough to bother back-patching it.

However, since commit 862ef372d6b, there *is* one test that fails if
you run make installcheck against a cluster running with -c
default_transaction_isolation=serializable: transaction.sql. Is that
a mistake? Is it a goal to be able to run this test suite against all
3 isolation levels?

@@ -1032,7 +1032,7 @@
SHOW transaction_isolation; -- out of transaction block
transaction_isolation
-----------------------
- read committed
+ serializable
(1 row)