test-oomd: fix flakiness under sanitizers

test-oomd: fix flakiness under sanitizers

The test asserts that pgscan is 0, but under sanitizers this sometimes
fails and shows up as 1. We cannot control what the kernel scans, and
with sanitizers the runtime can be slow enough it's possible that the
kernel does a pass on the cgroup of the unit test.
Instead of asserting that it's 0, assert that it's between 0 and 9,
which seems a reasonable range.

Fixes https://github.com/systemd/systemd/issues/37710