Fix NPE in TestRunSession#toString and use AtomicReference #2930

Job	Run time
check-freeze-period / verify-freeze-period	1s
build / Verify Linux	18m 4s
build / Upload Event File	3s
build / Verify Windows	26m 35s
check-merge-commits / Block Merge Commits	7s
build / Verify MacOS	6m 27s
	51m 17s

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