test: adjust MemoryHigh= on oomd extended test units

test: adjust MemoryHigh= on oomd extended test units

On some runs `sleep infinity` run by the user manager uses over 3M of
memory, which is higher than the MemoryHigh= set on testbloat and
testmunch. If no pgscan is generated, then systemd-oomd sorts by memory
usage which leads to a situation where testchill (using 3M) could be
targeted over testbloat (1M-2M).

Fix this by setting reasonable MemoryHigh= values for all of these test
units. Even if somehow testchill throttles a bit at 3M, testbloat and
testmunch should still be trying to use over 100M at memory and will
throttle down to 5M and 6M with the new values. This should reflect
the desired state in pgscan and memory usage during the test run.

Fixes #21684