pgsql: Improve test coverage for LISTEN/NOTIFY.

Improve test coverage for LISTEN/NOTIFY.

Back-patch commit b10f40bf0 into older branches.  This adds reporting
of NOTIFY messages to isolationtester.c, and extends the async-notify
test to include direct tests of basic NOTIFY functionality.

This provides useful infrastructure for testing a bug fix I'm about
to back-patch, and there seems no good reason not to have better tests
of LISTEN/NOTIFY in the back branches.  The commit's survived long
enough in HEAD to make it unlikely that it will cause problems.

Back-patch as far as 9.6.  isolationtester.c changed too much in 9.6
to make it sane to try to fix older branches this way, and I don't
really want to back-patch those changes too.

Discussion: https://postgr.es/m/31304.1564246011@sss.pgh.pa.us

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/295054411ebbff7ecfcc801ddfbdbadc1bac9f72

Modified Files
--------------
src/test/isolation/expected/async-notify.out | 97 ++++++++++++++++++++++++++--
src/test/isolation/isolationtester.c         | 88 +++++++++++++++----------
src/test/isolation/specs/async-notify.spec   | 76 +++++++++++++++++++---
3 files changed, 211 insertions(+), 50 deletions(-)