smtpsrv: Disable reloads during tests

Reloading during tests will cause the testing aliases to be removed,
which makes test runs that extend beyond 30s to be flaky.

This patch fixes the bug by disabling reloads during these tests.

internal/smtpsrv/server_test.go

@@ -584,6 +584,9 @@ func realMain(m *testing.M) int {
 		// Disable SPF lookups, to avoid leaking DNS queries.
 		disableSPFForTesting = true
 
+		// Disable reloading.
+		reloadEvery = nil
+
 		go s.ListenAndServe()
 	}