selftests: vm: drop dependencies on page flags from mlock2 tests

It was noticed that mlock2 tests are failing after 9c4e6b1a ("mm,
mlock, vmscan: no more skipping pagevecs") because the patch has changed
the timing on when the page is added to the unevictable LRU list and thus
gains the unevictable page flag.

The test was just too dependent on the implementation details which were
true at the time when it was introduced.  Page flags and the timing when
they are set is something no userspace should ever depend on.  The test
should be testing only for the user observable contract of the tested
syscalls.  Those are defined pretty well for the mlock and there are other
means for testing them.  In fact this is already done and testing for page
flags can be safely dropped to achieve the aimed purpose.  Present bits
can be checked by /proc/<pid>/smaps RSS field and the locking state by
VmFlags although I would argue that Locked: field would be more
appropriate.

Drop all the page flag machinery and conside...