libnetwork/resolvconf: allow tests to be run on unix

Looks like the intent is to exclude windows (which wouldn't have /etc/resolv.conf
nor systemd), but most tests would run fine elsewhere. This allows running the
tests on macOS for local testing.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libnetwork/resolvconf/resolvconf_linux_test.go → libnetwork/resolvconf/resolvconf_unix_test.go

@@ -1,3 +1,6 @@
+//go:build !windows
+// +build !windows
+
 package resolvconf
 
 import (