d/t/localtgt: reduce memory pressure avoiding OOM triggered false positives

Due to changes in fio the memory consumption increased a lot from ~450M to ~1.1G even for our trivial workload. This was discussed in an upstream issue 1 and a mitigation was mentioned.

This change adapts the test scripts in d/t/localtgt to use that dropping the memory pressure by two orders of magnitude and thereby avoiding false positives 2 that formerly got triggered by OOMs due to that.