chore: fail server startup on invalid DERP map (#10536)

cli/server_test.go

@@ -1755,3 +1755,22 @@ func TestConnectToPostgres(t *testing.T) {
 	})
 	require.NoError(t, sqlDB.PingContext(ctx))
 }
+
+func TestServer_InvalidDERP(t *testing.T) {
+	t.Parallel()
+
+	// Try to start a server with the built-in DERP server disabled and no
+	// external DERP map.
+	inv, _ := clitest.New(t,
+		"server",
+		"--in-memory",
+		"--http-address", ":0",
+		"--access-url", "http://example.com",
+		"--derp-server-enable=false",
+		"--derp-server-stun-addresses", "disable",
+		"--block-direct-connections",
+	)
+	err := inv.Run()
+	require.Error(t, err)
+	require.ErrorContains(t, err, "A valid DERP map is required for networking to work")
+}