Allow programs in the sandbox to write temp files.

This creates a tmp directory in the sandbox directory, and points
TMPDIR at it.  Allows writing files to that directory.
This test was misnamed, and is now skipped.

The test was written when I thought I could limit many small files. I've
changed it back to assert that the many files were prevented, but since
we don't prevent them yet, I've skipped the test.
Make a clearer distinction between argv and argv. :)
Merge pull request #12 from edx/ned/allow-tempfiles-in-sandbox

Allow programs in the sandbox to write temp files.