Test coverage for search_n with impossible value

[AlexGuteniev]

Thanks for adding some imposible coverage to vector algorithms, @StephanTLavavej!

But you seem to miss the one you've then-recently reviewed 😹

I had to damage the sequence in the test array a bit, because for search_n coverage to properly cover the right branch, we need to search for more than one consecutive element, otherwise we get special n=1 case.