test(util): MockProtect when mocking multiple times

It is fine to use MockVariableValue to change a setting such as:

defer test.MockVariableValue(&setting.Mirror.Enabled, true)()

But when testing for errors and mocking a function, multiple variants
of the functions will be used, not just one. MockProtect a function
will make sure that when the test fails it always restores a sane
version of the function. For instance:

defer test.MockProtect(&mirror_service.AddPushMirrorRemote)()

mirror_service.AddPushMirrorRemote = mockOne
do some tests that may fail

mirror_service.AddPushMirrorRemote = mockTwo
do more tests that may fail

(cherry picked from commit 884b3c0f4b)

modules/test/utils_test.go

@@ -0,0 +1,18 @@
+// Copyright 2024 The Forgejo Authors
+// SPDX-License-Identifier: MIT
+
+package test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMockProtect(t *testing.T) {
+	mockable := "original"
+	restore := MockProtect(&mockable)
+	mockable = "tainted"
+	restore()
+	assert.Equal(t, "original", mockable)
+}