Add regression test for bug fixed by recent refactoring.

Test case by Andres Freund for bug fixed by Tom Lane's refactoring
in commit 5194024d72f33fb209e10f9ab0ada7cc67df45b7
Fix permission tests for views/tables proven empty by constraint exclusion.

A view defined as "select <something> where false" had the curious property
that the system wouldn't check whether users had the privileges necessary
to select from it.  More generally, permissions checks could be skipped
for tables referenced in sub-selects or views that were proven empty by
constraint exclusion (although some quick testing suggests this seldom
happens in cases of practical interest).  This happened because the planner
failed to include rangetable entries for such tables in the finished plan.

This was noticed in connection with erroneous handling of materialized
views, but actually the issue is quite unrelated to matviews.  Therefore,
revert commit 200ba1667b3a8d7a9d559d2f05f83d209c9d8267 in favor of a more
direct test for the real problem.

Back-patch to 9.2 where the bug was introduced (by commit
7741dd6590073719688891898e85f0cb73453159).
Use correct length to convert json unicode escapes.

Bug reported on IRC - fix due to Andrew Gierth.
pg_test_fsync: update output to show usecs/op clearer
Prevent (auto)vacuum from truncating first page of populated matview.

Per report from Fujii Masao, with regression test using his example.