Ensure all stream buffers are drained before exiting process in single run mode.

After investigating a strange behavior on running karma with TeamCity on Windows, I noticed the following issue.

When running the following command in PowerShell:

```
karma start --single-run | Out-File C:\test.out
```

The writing function is non-blocking/asynchronous & the written/piped data can be buffered, which could lead to some test results being lost.

Karma needs to ensure that all buffers have been drained or that the writing functions have completed (if they are non-blocking) before exiting the process.

Start by reproducing the issue with `karma start --single-run | Out-File C:\test.out` in PowerShell on Windows, then trace the single-run process exit and output-writing paths. Done means the process waits for asynchronous or buffered output to finish so piped test results are not lost.

Written by the indexing model from the issue text.