MM-14559 Rewrite existing plugin API tests to exclusively test via actual plugin (#10494)

* initial work on rewriting test cases using the raw literal format

* split out the embedded tests into separate file and implement the test using a generic tester function

* initial work on rewriting test cases using the raw literal format

* split out the embedded tests into separate file and implement the test using a generic tester function

* moved tests to separate dir to avoid conflicts

* removed duplicate files

* added license header

* fixed plugin paths

* moved all trivial tests to separate files
created "meta-test" that scans a folder of api tests and runs them

* remove rpc tests from make

* use fileutil.FindDir instead of pwd
clean up the test files

tests/plugin_tests/test_get_direct_channel_plugin/main.go

@@ -0,0 +1,73 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package main
+
+import (
+	"reflect"
+
+	"github.com/mattermost/mattermost-server/model"
+	"github.com/mattermost/mattermost-server/plugin"
+)
+
+type MyPlugin struct {
+	plugin.MattermostPlugin
+}
+
+func isEmpty(object interface{}) bool {
+
+	// get nil case out of the way
+	if object == nil {
+		return true
+	}
+
+	objValue := reflect.ValueOf(object)
+
+	switch objValue.Kind() {
+	// collection types are empty when they have no element
+	case reflect.Array, reflect.Chan, reflect.Map, reflect.Slice:
+		return objValue.Len() == 0
+	// pointers are empty if nil or if the value they point to is empty
+	case reflect.Ptr:
+		if objValue.IsNil() {
+			return true
+		}
+		deref := objValue.Elem().Interface()
+		return isEmpty(deref)
+	// for all other types, compare against the zero value
+	default:
+		zero := reflect.Zero(objValue.Type())
+		return reflect.DeepEqual(object, zero.Interface())
+	}
+}
+
+func (p *MyPlugin) MessageWillBePosted(c *plugin.Context, post *model.Post) (*model.Post, string) {
+	dm1, err := p.API.GetDirectChannel("{{.BasicUser.Id}}", "{{.BasicUser2.Id}}")
+	if err != nil {
+		return nil, err.Error()
+	}
+	if isEmpty(dm1) {
+		return nil, "dm1 is empty"
+	}
+
+	dm2, err := p.API.GetDirectChannel("{{.BasicUser.Id}}", "{{.BasicUser.Id}}")
+	if err != nil {
+		return nil, err.Error()
+	}
+	if isEmpty(dm2) {
+		return nil, "dm2 is empty"
+	}
+
+	dm3, err := p.API.GetDirectChannel("{{.BasicUser.Id}}", model.NewId())
+	if err == nil {
+		return nil, "Expected to get error while fetching incorrect channel"
+	}
+	if !isEmpty(dm3) {
+		return nil, "dm3 is NOT empty"
+	}
+	return nil, ""
+}
+
+func main() {
+	plugin.ClientMain(&MyPlugin{})
+}