MM-2276: Fix flaky TestPlugin in cmd/mattermost/commands (#14064)

There was a race in (*App).SyncPlugins where if the same plugin
existed in availablePlugins and pluginSignaturePathMap, then
we would try to add/remove at the same time.

This would lead to a possible removal / addition of a plugin directory
or even unable to remove a directory because it was already in use.

We fix this by first finishing the removal of availablePlugins
before syncing it with the file store.

app/plugin.go

@@ -230,6 +230,7 @@ func (a *App) SyncPlugins() *model.AppError {
 			}
 		}(plugin.Manifest.Id)
 	}
+	wg.Wait()
 
 	// Install plugins from the file store.
 	pluginSignaturePathMap, appErr := a.getPluginsFromFolder()
@@ -263,7 +264,6 @@ func (a *App) SyncPlugins() *model.AppError {
 				mlog.Error("Failed to sync plugin from file store", mlog.String("bundle", plugin.path), mlog.Err(err))
 			}
 		}(plugin)
-
 	}
 
 	wg.Wait()