Make PageCacheMonitor test less flaky

The new possibility for blocking page faults and direct transfer of new pages,
make it possible for more evictions to happen. These tests were written when
this wasn't the case, so they didn't take this into account. This has been
fixed.
Merge pull request #3212 from chrisvest/2.2-flaky-pagecache-monitor-test

[2.2] Make PageCacheMonitor test less flaky