valgrind errors occur too often when 'make test' is used. It is because too

many third-party libs and tools have problems. When curl is built without
--disable-shared, the testing is done with a front-end script which makes the
valgrind testing include (ba)sh as well and that often causes valgrind
errors. Either we improve the valgrind error scanner a lot to better identify
(lib)curl errors only, or we disable valgrind checking by default