kunit: tool: misc fixes (unused vars, imports, leaked files)

Drop some variables in unit tests that were unused and/or add assertions
based on them.

For ExitStack, it was imported, but the `es` variable wasn't used so it
didn't do anything, and we were leaking the file objects.
Refactor it to just use nested `with` statements to properly close them.

And drop the direct use of .close() on file objects in the kunit tool
unit test, as these can be leaked if test assertions fail.

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