Use cl_assert_equal_s() instead of strcmp().

In one former PR I used the pattern cl_assert(!strcmp()) for testing for string equality because I observed the pattern in the current code. It was suggested to use cl_assert_equal_s(), which is used more prominently. In this RP, all cl_assert(!strcmp()) or semantically equivalent forms are replaced by cl_assert_equal_s() so newbies no longer will observe the wrong pattern :)