Add interval parameter to assert.retry

assert.retry (https://github.com/mongodb/mongo/blob/f5c613f11f170cadd4b698d108b3c7b636913568/src/mongo/shell/assert.js#L223-L235) currently does not wait between retries. While this is appropriate default behavior, most uses of it likely want to wait some interval between retries. We should add an interval to sleep between retries with a default of 0.