tests: lib: cbprintf_package: Extend test coverage

Add test for cbprintf_package_convert function which checks if
it correctly handles array that holds string lengths. When convert
function is used twice, at first to calculate size of the output
package and then to actually convert the package, array of string
lengths can be used to optimize operation by not calculating
string lengths twice. However, array may not be able to hold all
string lengths that are needed for that package. In that case,
string lengths that did not fit into the array will be calculated
twice, in both conversions.

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