Reworked WrapperConcurrentDownloadTest so that it runs real wrapper processes.
- Changed wrapper install so that the lock is held while checking if the distribution has already been downloaded, to handle the case where another process is currently unzipping the distribution.
- Don't release the lock until after downloading and unzipping the distribution, to avoid multiple processes downloading the same distribution.
- Don't delete the lock file when releasing the lock, to handle the case where another process is currently trying to acquire the lock.