spec: isolate plugin settings better in specs

fixes brittle ./spec/lib/app_center/app_api_spec.rb:112 and possibly
others. if #current_account is meaningful and gets set in one spec, make
sure it doesn't affect subsequent specs (in particular ones that use
caching)

Change-Id: I2b54fcf8e941a8d3a50c367f0f1d8e18d018d00b
Reviewed-on: https://gerrit.instructure.com/68784
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>

app/models/plugin_setting.rb

@@ -142,4 +142,7 @@ class PluginSetting < ActiveRecord::Base
   def self.find_by_name(name)
     where(name: name).first
   end
+
+  # stub for plugins who want to do something fancy
+  def self.current_account=(_); end
 end

spec/spec_helper.rb

@@ -448,6 +448,7 @@ RSpec.configure do |config|
     Time.zone = 'UTC'
     LoadAccount.force_special_account_reload = true
     Account.clear_special_account_cache!(true)
+    PluginSetting.current_account = nil
     AdheresToPolicy::Cache.clear
     Setting.reset_cache!
     ConfigFile.unstub