docker: test-debug: disable LeakSanitizer

docker: test-debug: disable LeakSanitizer

There are just too many leaks in device-introspect-test (especially for
the plethora of arm and aarch64 boards) to make LeakSanitizer useful;
disable it for now.

Whoever is interested in debugging leaks can also use valgrind like this:

   QTEST_QEMU_BINARY=aarch64-softmmu/qemu-system-aarch64 \
   QTEST_QEMU_IMG=qemu-img \
   valgrind --trace-children=yes --leak-check=full \
   tests/device-introspect-test -p /aarch64/device/introspect/concrete/defaults/none

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>