Improve doctor cli behavior (#28422)

1. Do not sort the "checks" slice again and again when "Register", it
just wastes CPU when the Gitea instance runs
2. If a check doesn't exist, tell the end user
3. Add some tests

cmd/doctor_test.go

@@ -0,0 +1,33 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package cmd
+
+import (
+	"context"
+	"testing"
+
+	"code.gitea.io/gitea/modules/doctor"
+	"code.gitea.io/gitea/modules/log"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/urfave/cli/v2"
+)
+
+func TestDoctorRun(t *testing.T) {
+	doctor.Register(&doctor.Check{
+		Title: "Test Check",
+		Name:  "test-check",
+		Run:   func(ctx context.Context, logger log.Logger, autofix bool) error { return nil },
+
+		SkipDatabaseInitialization: true,
+	})
+	app := cli.NewApp()
+	app.Commands = []*cli.Command{cmdDoctorCheck}
+	err := app.Run([]string{"./gitea", "check", "--run", "test-check"})
+	assert.NoError(t, err)
+	err = app.Run([]string{"./gitea", "check", "--run", "no-such"})
+	assert.ErrorContains(t, err, `unknown checks: "no-such"`)
+	err = app.Run([]string{"./gitea", "check", "--run", "test-check,no-such"})
+	assert.ErrorContains(t, err, `unknown checks: "no-such"`)
+}