support/testing: add missing systemd-overlay-fstab

Commit 083e65a67c84f83a3403c92c51d89f8c94907c55 introduced tests for the
various read-only root options under systemd, but while applying the
fs-overlay that is used in one of the tests wasn't included. Include it
now.

Fixes: 083e65a67c84f83a3403c92c51d89f8c94907c55
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

support/testing/tests/init/systemd-overlay-fstab/etc/fstab

@@ -0,0 +1,2 @@
+/dev/root / auto ro 0 1
+other-var-backing-store /run/buildroot/mounts/var tmpfs defaults