lib: cbprintf: add unit tests for deferred formatting

Tests to exercise the new `cbprintf_package()`, `cbvprintf_package()`
and `cbpprintf()`.

[ Heavily based on a prior proposal from Peter Bigot. ]

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>