resurrect valgrind testing

Since valgrind checking takes many hours, it's simply not feasible for it to be part of the normal implement-MR-test-investigate-debug cycle. So it's active only on the master and valgrind branches.

Note: valgrind testing can be triggered by hand with:

DEBUG=1 CI_BUILD_NAME=test TIMEOUT_SCALE=8 valgrind=1 ci/40-testsuite.sh

(the TIMEOUT_SCALE here acts multiplicatively on the configured timeout values for tests that are defined based on normal, non-valgrind reasonable limits)