pgsql: Add an SSI regression test that tests all interesting permutatio

Add an SSI regression test that tests all interesting permutations in the
order of begin, prepare, and commit of three concurrent transactions that
have conflicts between them.

The test runs for a quite long time, and the expected output file is huge,
but this test caught some serious bugs during development, so seems
worthwhile to keep. The test uses prepared transactions, so it fails if the
server has max_prepared_transactions=0. Because of that, it's marked as
"ignore" in the schedule file.

Dan Ports

Branch
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/b9585e8f983863fc7e1e147f503091a5f82d6ccf

Modified Files
--------------
.../isolation/expected/prepared-transactions.out |34843 ++++++++++++++++++++
src/test/isolation/isolation_schedule | 5 +
src/test/isolation/isolationtester.c | 12 +-
.../isolation/specs/prepared-transactions.spec | 1507 +
4 files changed, 36365 insertions(+), 2 deletions(-)