logging: Fix false strdup buffer freeing

Algorithm for freeing strdup buffers was only checking if argument
matches address within strdup buffer pool and was attempting freeing
even if format specifier was different than string.

Added fix where also format specifier is checked.

Extended logger test to verify correctness of function which searches
for string format specifiers within a string.

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