support/testing: TestInitSystemSystemdRwIfupdown test expect a RW rootfs

support/testing: TestInitSystemSystemdRwIfupdown test expect a RW rootfs

When running the TestInitSystemSystemdRwIfupdown test, the rootfs must
be in read-write mode. The commit log [1] introducing systemd tests say
so:

"basic systemd, read-write, network w/ ifupdown"

With systemd 246.5, the service systemd-update-done return an error code
when it can't write on the filesystem (/etc)

[1] 117835d5fcd508f301d62dd08ee658c1982c7fa7
[2] https://github.com/systemd/systemd/commit/8019995e9af9c6d7b5985198cedccd24eda3e26e

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/830981813

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>