pgsql: Allow using Unix-domain sockets on Windows in tests

Allow using Unix-domain sockets on Windows in tests

The test suites currently don't use Unix-domain sockets on Windows.
This optionally allows enabling that by setting the environment
variable PG_TEST_USE_UNIX_SOCKETS.

This should currently be considered experimental.  In particular,
pg_regress.c contains some comments that the cleanup code for
Unix-domain sockets doesn't work correctly under Windows, which hasn't
been an problem until now.  But it's good enough for locally
supervised testing of the functionality.

Reviewed-by: Andrew Dunstan <andrew.dunstan@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/flat/54bde68c-d134-4eb8-5bd3-8af33b72a010@2ndquadrant.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1d53432ff940b789c2431ba476a2a6e2db3edf84

Modified Files
--------------
src/bin/pg_ctl/t/001_start_stop.pl        |  2 +-
src/test/authentication/t/001_password.pl |  7 +++---
src/test/authentication/t/002_saslprep.pl |  7 +++---
src/test/perl/PostgresNode.pm             |  4 ++--
src/test/perl/TestLib.pm                  |  8 ++++++-
src/test/regress/pg_regress.c             | 40 ++++++++++++++++++++-----------
6 files changed, 42 insertions(+), 26 deletions(-)