pgsql: Make new isolationtester test more stable

Make new isolationtester test more stable

The original coding of the test was relying too much on the ordering in
which backends are awakened once an advisory lock which they wait for is
released.  Change the code so that each backend uses its own advisory
lock instead, so that the output becomes stable.  Also add a few seconds
of sleep between lock releases, so that the test isn't broken in
overloaded buildfarm animals, as suggested by Tom Lane.

Per buildfarm members spoonbill and guaibasaurus.

Discussion: https://www.postgresql.org/message-id/19294.1456551587%40sss.pgh.pa.us

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/54638f5708e59e5fd7c79b8315d8c6e810d7bc13

Modified Files
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src/test/isolation/expected/tuplelock-update.out | 26 +++++++++++++++++-------
src/test/isolation/specs/tuplelock-update.spec   | 16 ++++++++++-----
2 files changed, 30 insertions(+), 12 deletions(-)