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