lib/oeqa/selftest/oescripts: make test_cleanup_workdir use selftest-ed

lib/oeqa/selftest/oescripts: make test_cleanup_workdir use selftest-ed

Use a recipe which is bundled in the meta-selftest layer for this test,
rather than relying on OE-Core remaining static (or updating the tests
when OE-Core changes recipes).

Tested with oe-selftest -r oescripts.TestScripts.test_cleanup_workdir

(From OE-Core rev: f8aabeb1a755f3312782a7b64fe863c155510b33)

Signed-off-by: Joshua Lock <joshua.g.lock@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>