kunit: configs: Enable coverage of the bitfield packing library

There are KUnit tests for the bitfield packing library but these depend
on CONFIG_PACKING which is not enabled by anything in either the
existing KUnit all_tests.config or the base UML config it runs on as
standard.  Enable that in all_tests.config to improve coverage.

Signed-off-by: Mark Brown <broonie@kernel.org>