By comparing only megabytes in the memory changes, instead of bytes as
before, since it may be tricky at least due to max_untracked_memory and
how thread pool handle it.
It should be safe, since originally it was written in #16121 which fixes
issue #15932, which has ~4MB consumption of memory per request.
01540_MemoryTracking is failing on CI for the following reasons:
- log_queries (fixed, by adding log_queries=0)
- profilers (fixed)
- but what can't be fixed is metric_log and so on, so we need separate
instance with separate configuration (sigh).
