tests: logging: log_core: Extended test_log_strdup_gc

Extended test to pass address within strdup buffer but with
different format specifier (not string). That should not trigger
string buffer freeing. If it does system may collapse (e.g. cortex-m0
may use unaligned access).

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