Process atests: Avoid handing non-terminable processes.

It seems Jython 2.7 isn't able kill process that don't terminate
cleanly. This change to tests avoid process left hanging in that case.
Related to #1826.
Enhance `Terminate Process` with Jython (#1826)

Catch error when trying to read from PIPE (our default stream) after
process has been terminated.  The error id due to JVM limitation
explained at http://bugs.jython.org/issue2218 and seems to occur
on non-Windows platforms.

In addition to the code fix, also this was done:

- Enhanced tests to verify that stuff written to streams before
  termination can be read correctly with Python and that we get
  an empty string with Jython outside Windows.
- Added tests that when using custom streams, reading stuff written
  before termination always works.
- Documented the limitation.