tests: lib: cbprintf_package: Rework C++ test

Reworked test to use different approach for C++ testing.
Instead of hackish include of test.inc, cmake is forcing
C++ compilation on test file when C++ option is set in
test configuration.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
5 files changed