Remove stale comments about missing test suite parts
    
    Parts of the test suite were missing in the official 1.5.5 release.
    This was addressed in 62c46b2 ("Fix FTBFS.", 2017-06-05) and fixed
    upstream in fd602b75 ("include entire test suite in gem (PR #1952)",
    2016-11-29).  The 1.5.6 release included that fix, but the comment
    remained after 46fa0bd ("Update to 1.5.6.1", 2017-08-24).