Race condition in SandboxPool

[nicojs]

There is a race condition in the SandboxPool. If the problem occurs, the SandboxPool will already signal that it is done disposing when there are actually still sandbox processes starting. This can result in the following error being logged:

[2018-04-20 14:23:16.837] [ERROR] TestRunnerChildProcessAdapter - C:\z\test\tsnodemocha\node_modules\stryker\src\isolated-runner\IsolatedTestRunnerAdapterWorker.js:59
        process.chdir(message.runnerOptions.sandboxWorkingFolder);

After stryker reports that it should be done.