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)