tests: shell: Implement --valgrind mode

tests: shell: Implement --valgrind mode

Wrap every call to $XT_MULTI with valgrind, or actually a wrapper script
which does the valgrind wrap and stores the log if it contains something
relevant.

Carefully name the wrapper script(s) so that test cases' checks on
$XT_MULTI name stay intact.

This mode slows down testsuite execution horribly. Luckily, it's not
meant for constant use, though.

For now, ignore commands with non-zero exit status - error paths
typically hit direct exit() calls and therefore leave reachable memory
in place.

Signed-off-by: Phil Sutter <phil@nwl.cc>