TestProcess: Improve reading from a pipe with a sub-process

[spevans]

• TestProcess.runTask() uses a pipe to read the stdout of a subprocess
  during tests however sometimes the data read from the pipe is
  truncated causing issues with the testing of the response.

• Use .readabilityHandler to read the data whilst the sub-process is
  running and then drain the pipe after the process has terminated.

[spevans]

@swift-ci test

[spevans]

@swift-ci test

[millenomi]

I'm so glad this works now.

[spevans]

@millenomi One thing I found when using the new FileHandle API is that there is no throwing equivalent to .availableData, ie nothing that ends up calling _readDataOfLength(count, untilEOF: false) which is a shame. One possible fix might be to modify the signature of read(upToCount count: Int) to read(upToCount count: Int, untilEOF: Bool = true) or some such.