oeqa: don't litter /tmp with temporary directories
If we need to create a temporary directory in targetbuild or buildproject use
tempfile.TemporaryDirectory so that when the test case is finished, the
directory is deleted.
Also synchronise the logic and don't possibly store the temporary directory in
self.tmpdir as nothing uses that.
Signed-off-by: Ross Burton <ross.burton@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
