Fold PythonTestBuilder into the PytestRun task.

Review Request #1993 - Created March 26, 2015 and submitted

Benjy Weinberger
jsirois, patricklaw
PythonTestBuilder was a vestige of the old, pre-task Pants. The PytestRun
task was implemented as a thin wrapper around it.  This change merges
the functionality into PytestRun and gets rid of PythonTestBuilder as
a standalone class.

- Fixes various kludges, such as logging:  Now code that was formerly
  in PythonTestBuilder has access to the task's logging facilities, so
  it can use those instead of print().
- PythonTestBuilder's test is now a task test for PytestRun. It uses
  the new TaskTestBase, and demonstrates how this base class could
  be used instead of the old PythonTaskTest in the remaining tests
  that still use the latter.

CI passed:

Benjy Weinberger
John Sirois
Benjy Weinberger
Benjy Weinberger
Review request changed

Status: Closed (submitted)

Change Summary:

Submitted as bbe4861ab6cace2f54002c37d8773732a72a77bd.