test: fix TEST-56-OOMD thresholds for linux 5.9 changes

test: fix TEST-56-OOMD thresholds for linux 5.9 changes

Fixes #17533

The memory pressure values of the units in TEST-56-OOMD seemed to be a
lot lower after updating to linux 5.9. This is likely due to a fix from
https://github.com/torvalds/linux/commit/e22c6ed90aa91abc08f107344428ebb8c2629e98.

To account for this, I lowered memory.high on testbloat.service to
throttle it even more. This was enough to generate the 50%+ value to trigger
oomd for the test, but as an extra precaution I also lowered the oomd
threshold to 1% so it's certain to try and kill testbloat.service.