Tests

Refactor and parallelize MaxText test runner #25

Job	Run time
prelim	8s
gpu_image / Build and upload image (a100-40gb-4)	14m 1s
tpu_image / Build and upload image (v4-8)	8m 0s
tpu_integration_tests / run	6m 1s
tpu_unit_tests / run	14m 38s
gpu_unit_tests / run	2m 54s
gpu_integration_tests / run	10m 4s
Clean up	8s
	55m 54s

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