kunit: string-stream: Simplify resource use

Currently, KUnit's string streams are themselves "KUnit resources".
This is redundant since the stream itself is already allocated with
kunit_kzalloc() and will thus be freed automatically at the end of the
test.

string-stream is only used internally within KUnit, and isn't using the
extra features that resources provide like reference counting, being
able to locate them dynamically as "test-local variables", etc.

Indeed, the resource's refcount is never incremented when the
pointer is returned. The fact that it's always manually destroyed is
more evidence that the reference counting is unused.

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