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