added simple unit test for multisearch_naive

make test
./u-multisearch_naive
    pass     21		are equal:     	results.size()	 ==  3
    pass     22		check:        	results == (vector<pos_t>{content.begin()+0, content.begin()+4, content.begin()+8})
    pass     36		are equal:     	results.size()	 ==  0

------------ all pass ------------