Re-enable TAP tests of pg_receivewal for ZLIB on Windows

This is a revert of 6cea447, that disabled those tests temporarily on
Windows due to failures with bowerbird where gzflush() would fail when
executed on a freshly-opened compressed and partial segment.  This
problem should be taken care of now thanks to 7fbe0c8, so let's see what
the buildfarm has to say on Windows for those tests.

Discussion: https://postgr.es/m/YPDLz2x3o1aX2wRh@paquier.xyz

src/bin/pg_basebackup/t/020_pg_receivewal.pl

@@ -72,13 +72,11 @@ $primary->command_ok(
 my @partial_wals = glob "$stream_dir/*\.partial";
 is(scalar(@partial_wals), 1, "one partial WAL segment was created");
 
-# Check ZLIB compression if available.  On Windows, some old versions
-# of zlib can cause some instabilities with this test, so disable it
-# for now.
+# Check ZLIB compression if available.
 SKIP:
 {
-	skip "postgres was not built with ZLIB support, or Windows is involved", 5
-	  if (!check_pg_config("#define HAVE_LIBZ 1") || $windows_os);
+	skip "postgres was not built with ZLIB support", 5
+	  if (!check_pg_config("#define HAVE_LIBZ 1"));
 
 	# Generate more WAL worth one completed, compressed, segment.
 	$primary->psql('postgres', 'SELECT pg_switch_wal();');