The concurrent gate access test is flaky since it depends on the order
of operations of two concurrent goroutines. Despite the heavy bias
towards one of the results, it's still possible to end the execution
with a closed gate.

I believe this case was created to test an earlier implementation where
the gate state was stored and mutated internally, so the access had to
be protected by a lock. However, the final implementation changed this
approach to be only channel-based, so there is no need for this flaky
test anymore.