chore(bigframes): move the load test script under .kokoro/continuous/ (#17225)

This is to make sure the test still runs even when there has been no
changes:


https://github.com/googleapis/google-cloud-python/blob/3ef95d61995869318097e414e439da1d6c214d1f/.kokoro/system.sh#L156-L161

Tracking issue: b/509623551

Author: sycai