add a few docs for plugin testing (#7798)

* add a few docs for plugin testing

* fix typo

plugin/example_test.go

@@ -0,0 +1,35 @@
+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))
+}