Use Civi::$statics instead of php static

This was driving me nuts trying to figure out why a test was failing. In tests, static vars carry over between tests. Using Civi::$statics it gets cleared between tests but otherwise functions the same.

• https://docs.civicrm.org/dev/en/latest/extensions/#interacting-with-core
• https://civicrm.org/blog/totten/the-static-is-dead-long-live-the-static