Fix flaky test TestGetSpawnToken_CommandStats

TestGetSpawnToken_CommandStats is flaky as it has a ten second timeout on acquiring a spawn token. In slower execution, for example in the CI, this timeout can be reached and the test flake.

There's no real reason to set a separate timeout here. We can rely on the general testing timeout after which the testing run ends. Remove the timeout. Additionally, replace the sleep with a call to Gosched to achieve the result we're after without sleeping.

Closes #6199 (closed)