oe-selftest: Add track_for_cleanup method to be used in cleanup tasks

oe-selftest: Add track_for_cleanup method to be used in cleanup tasks

Added a track_for_cleanup(path) method that removes the given path in the
tearDown method. This mechanism can be used to make sure a file or directory
we created will be removed at the end of a test, regardless of what happens.

(From OE-Core rev: 358415cf604089cc2dab547e231d062b9dc068ee)

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>