avoiding use of popen.

os.popen(). Fixes #692222.

imports of test modules now import from the test package.  Other
related oddities are also fixed (like DeprecationWarning filters that
weren't specifying the full import part, etc.).  Also did a general
code cleanup to remove all "from test.test_support import *"'s.  Other
from...import *'s weren't changed.

existed at the time atexit first got imported.  That's a bug, and this
fixes it.

Also reworked test_atexit.py to test for this too, and to stop using
an "expected output" file, and to test what actually happens at exit
instead of just simulating what it thinks atexit will do at exit.

Bugfix candidate, but it's messy so I'll backport to 2.2 myself.

'verify' iff it's used by a test module...

and replaces them with a new API verify(). As a result the regression
suite will also perform its tests in optimization mode.

Written by Marc-Andre Lemburg. Copyright assigned to Guido van Rossum.

<skip@mojam.com>.  Revisions to the markup to make it pass LaTeX, added
an index entry and a reference from the sys.exitfunc documentation.

This closes SourceForge patch #100620.