oe-selftest: implement add_command_to_tearDown method

oe-selftest: implement add_command_to_tearDown method

Add a new method that can be used by the tester to add a command to the
executed in the tearDown stage of the test. This mechanism can be used to
make sure certain test-specific cleanup tasks are done in the case of a
test failure.

(From OE-Core rev: b59466ec341e6596b7ade7f1813b25e454b11a32)

Signed-off-by: Corneliu Stoicescu <corneliux.stoicescu@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>