test-oomd-util: remove memory_pressure == 0 checks

test-oomd-util: remove memory_pressure == 0 checks

test_oomd_cgroup_context_acquire_and_insert reads the live cgroup data used
by the unit test. Under certain conditions, the memory pressure for the cgroup
can be non-zero (although most of the time it is 0 since these tests don't
generate much pressure).

Since these values are too dependent on the state of the system, remove the
checks. The type used is always >= 0 and test-psi-util already unit tests that
PSI values are parsed correctly from files so this test is redundant anyways.