Exhaustive test runs on CentOS 6 can take around 20-22 hours even in the normal case.
run-all-tests-timeout-check.sh installs a timeout guard of 72000 s, i.e. 20 hours, which includes only the tests, but excludes build and dataload time.
On recent runs some of the spilling tests ran slowly but successfully, which made the timeout fire during the last items of custom_cluster_tests, cutting the test run short.
I think the timeout should be made longer for CentOS 6 based tests until we find ways to run those faster.