pgsql: Allow pg_regress.c wrappers to postprocess test result files.

Allow pg_regress.c wrappers to postprocess test result files.

Add an optional callback to regression_main() that, if provided,
is invoked on each test output file before we try to compare it
to the expected-result file.

The main and isolation test programs don't need this (yet).
In pg_regress_ecpg, add a filter that eliminates target-host
details from "could not connect" error reports.  This filter
doesn't do anything as of this commit, but it will be needed
by the next one.

In the long run we might want to provide some more general,
perhaps pattern-based, filtering mechanism for test output.
For now, this will solve the immediate problem.

Discussion: https://postgr.es/m/BN6PR05MB3492948E4FD76C156E747E8BC9160@BN6PR05MB3492.namprd05.prod.outlook.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/800d93f314b0f7c10193e48b259f87800cb85d84

Modified Files
--------------
src/interfaces/ecpg/test/pg_regress_ecpg.c | 95 +++++++++++++++++++++++++++---
src/test/isolation/isolation_main.c        |  5 +-
src/test/regress/pg_regress.c              | 29 +++++----
src/test/regress/pg_regress.h              | 24 ++++++--
src/test/regress/pg_regress_main.c         |  5 +-
5 files changed, 134 insertions(+), 24 deletions(-)