kunit_tool: added tests for throw_error_if_not_subset() Added tests to make sure method correctly throws exception if the first param is not a subset of the second param. Signed-off-by: Maria Del Carmen Ignacio <mariaignacio@google.com> Signed-off-by: Darya Verzhbinsky <daryaver@google.com> Change-Id: I651d96fbb8e59be03eba82bb2330336018493f6e