python/tests: add TestCaseInTempdir.mktemp()

python/tests: add TestCaseInTempdir.mktemp()

This gives you a name of a temporary file within the test case's tempdir.
Use it like this:

  with self.mktemp() as filename:
     self.check_run("samba-tool foo --output %s" % filename)
     self.assertStringsEqual(open(filename).read(), expected)

and filename will flick out of existence when the with block ends.

This is based on an idea used in the traffic_runner tests, which will
soon be adapted to use this method.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>