buildman: Create temp directory in test setup

buildman: Create temp directory in test setup

Rather than having a few tests handle this themselves, create the
temporary directory in the setUp() method and remove it in tearDown().
This will make it easier to add more tests.

Only testOutput and testGit() actually need it, but it doesn't add to the
test time noticeably to do this for all tests in this file.

Signed-off-by: Simon Glass <sjg@chromium.org>