The creation of parent directories is asynchronous process, which was not properly awaited, which resulted in the coverage reports not being written to the disk sometimes as Karma process has exited before the reporter completed the writing of the coverage report.

Also update the test case to use async implementation of the corresponding stub to prevent regressions in the future. Remove manual calls for `done` parameter in tests as it is handled by the stub now.

Fixes karma-runner#434

The issue surfaced itself after d970028. The EEXIST error from concurrent attempts to create the same directory were previously silently swallowed, but started to show up once the call was properly synchronized.

The EEXIST will now result in `promiseComplete` being rejected and reported as `unhandledRejection` because it is awaited in the `onExit` callback. The unhandled rejection is then picked up by `karma` [here](https://github.com/karma-runner/karma/blob/c985155a4eac95c525e1217e98d4013ac5f53305/lib/server.js#L395) triggering [the close logic](https://github.com/karma-runner/karma/blob/c985155a4eac95c525e1217e98d4013ac5f53305/lib/server.js#L392) which (among other things) will trigger `onExit` callback causing an infinite loop.

The local fix is to handle the rejected promise directly and report failure to karma by passing a non-zero exit code.