killed stub would not record execution

[toote]

Unstubbing fails when running something like the following:

stub $COMMAND "sleep 60 & wait \$!"
run $COMMAND &

kill $!

unstub $COMMAND

The reason stubbing fails is because as the command itself was killed when running, there was no accounting done on the call. Obviously, doing this is not a common scenario (nor killing commands in scripts just like that).

Context: found (same as #14) when testing concurrent executions of uploads in the BuildKite test-collector plugin