tests: Disable valgrind for core dumping and for self-introspecting tests.

tests: Disable valgrind for core dumping and for self-introspecting tests.

The only tests that fork and exec are those that will then try to ptrace
attach the child for unwind testing. That won't work when the child is
also running under valgrind. So remove --trace-children=yes. Also disable
valgrind while dumping a core or for tests that try to inspect their own
state. They will get confused otherwise finding pieces of valgrind in
their maps.

Note we still seem to hit the following valgrind bug because the way we
map and unmap some shared libraries gets valgrind confused:
https://bugs.kde.org/show_bug.cgi?id=327427

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