Complements e2d6e146.

914c72be introduced a regression that
causes saving a NamedTemporaryFile in a FileField to raise a
SuspiciousFileOperation. To remedy this, if a File has an absolute
path as a filename, use only the basename as the filename.

Complements 47f22e82.

Previously, empty values were saved as strings.

If settings.FILE_UPLOAD_TEMP_DIR is the default value of None,
NamedTemporaryFile will work the same as if nothing was passed.

The fact that we aren't dealing with the Django source tree anymore
allows us to drop several tearDown()/addCleanup() calls that were
concerned with removing apiece files/dirs/symlinks created by test
cases, as we are covered by the removal of the parent temporary tree
anyways.

Thanks Tim Graham for advice and review.

This allows makemessages/compilemessages tests in `test_extraction.py`
and `test_compilation.py` to actually run isolated from each other
(unaffected by stray FS objects left by cleanup actions failures, debug
sessions, etc.) and to take advantage of the parallel tests execution
feature like most of the Django test suite.

`test_percents.py` gets slightly refactored to not inherit from the new
machinery which sets up every test case to copy and run under a
temporary tree.

Thanks Alex Morozov for the initial patch.

Thanks ArcTanSusan for the initial patch.