support/testing/tests/init/test_systemd.py: use downloaded kernel

Since systemd requires a relatively new kernel, we switched to a
self-built 4.19.204 kernel. However, since then, the downloaded kernel
used by the tests has been updated to 5.10, which is certainly recent
enough to support systemd. Switch to this one. This reduces the test
time significantly.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>