tests: Use valgrind --leak-check=full.

When --enable-valgrind is given run the testsuite with --leak-check=full
treating any memory leak as a failure. Also disable valgrind for those
tests that trigger a valgrind bug.
https://bugs.kde.org/show_bug.cgi?id=327427



Signed-off-by: Mark Wielaard <mjw@redhat.com>