kunit: tool: fix unit test cleanup handling

* Stop leaking file objects.
* Use self.addCleanup() to ensure we call cleanup functions even if
setUp() fails.
* use mock.patch.stopall instead of more error-prone manual approach

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Acked-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>