kunit: drop test pointer in string_stream_fragment

We already store the `struct kunit *test` in the string_stream object
itself, so we need don't need to store a copy of this pointer in every
fragment in the stream.

Drop it, getting string_stream_fragment down the bare minimum: a
list_head and the `char *` with the actual fragment.

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>