[Python] test_plasma could make errors more diagnosable

[asfimport]

Currently, when plasma_store fails for a reason or another, you get poorly readable errors from test_plasma.py. Displaying the child process' stderr would help.

Reporter: Antoine Pitrou / @pitrou

_{Note: This issue was originally created as ARROW-2271. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Sorry for this erroneous report. stderr is displayed by py.test (it is simply passed through from the child to the parent). I blame looking at two bugs at once :-)