--source include/wait_condition.inc very slow in main.show_explain

Affects only macOS afaik. main.show_explain includes wait_condition.inc whenever it needs to wait for a bit before checking a condition. On linux, this wait is very short but on macOS (as verified with --mark-progress), it's quite lengthy, sometimes 10 or more seconds. Why is this? Interestingly, the $wait_condition variable isn't set before sourcing wait_condition.inc; setting that variable is a common pattern in other tests that use wait_condition.inc. Perhaps that usage is incorrect and on Linux it just happens to be enough of a delay for the desired event to occur, but I'm speculating.