add a few docs for plugin testing (#7798)

* add a few docs for plugin testing

* fix typo

plugin/plugintest/plugintest.go

@@ -0,0 +1,45 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+// The plugintest package provides mocks that can be used to test plugins. For example, to test the
+// ServeHTTP method of the plugin package's HelloUser example:
+//
+//    package plugin_test
+//
+//    import (
+//        "io/ioutil"
+//        "net/http/httptest"
+//        "testing"
+//
+//        "github.com/stretchr/testify/assert"
+//        "github.com/stretchr/testify/require"
+//
+//        "github.com/mattermost/mattermost-server/model"
+//        "github.com/mattermost/mattermost-server/plugin/plugintest"
+//    )
+//
+//    func TestHelloUserPlugin(t *testing.T) {
+//        user := &model.User{
+//            Id:       model.NewId(),
+//            Username: "billybob",
+//        }
+//
+//        api := &plugintest.API{}
+//        api.On("GetUser", user.Id).Return(user, nil)
+//        defer api.AssertExpectations(t)
+//
+//        p := &HelloUserPlugin{}
+//        p.OnActivate(api)
+//
+//        w := httptest.NewRecorder()
+//        r := httptest.NewRequest("GET", "/", nil)
+//        r.Header.Add("Mattermost-User-Id", user.Id)
+//        p.ServeHTTP(w, r)
+//        body, err := ioutil.ReadAll(w.Result().Body)
+//        require.NoError(t, err)
+//        assert.Equal(t, "Welcome back, billybob!", string(body))
+//    }
+//
+// The mocks are created using testify's mock package:
+// https://godoc.org/github.com/stretchr/testify/mock
+package plugintest