smtpsrv: Disable SPF lookups in tests

When testing, we don't want the server to do SPF lookups, as those cause
real DNS queries which can be problematic and add a dependency on the
environment.

This patch adds an internal boolean to disable the SPF lookups, which is
only set from the tests.

internal/smtpsrv/server_test.go

@@ -453,6 +453,9 @@ func realMain(m *testing.M) int {
 		s.AddDomain("localhost")
 		s.AddUserDB("localhost", udb)
 
+		// Disable SPF lookups, to avoid leaking DNS queries.
+		disableSPFForTesting = true
+
 		go s.ListenAndServe()
 	}