ReSharper is wrong. You test what happens when the configuration contains an invalid value; but you never test what happens when the configuration contains a valid value.
There are four paths you need to test:
CacheOptions:Menu:SlidingExpiration
is invalid, CacheOptions:Menu:AbsoluteExpiration
is invalid;CacheOptions:Menu:SlidingExpiration
is invalid, CacheOptions:Menu:AbsoluteExpiration
is valid;CacheOptions:Menu:SlidingExpiration
is valid, CacheOptions:Menu:AbsoluteExpiration
is invalid;CacheOptions:Menu:SlidingExpiration
is valid, CacheOptions:Menu:AbsoluteExpiration
is valid;