Fix bit-rotted planner test case.

While fooling with my pet outer-join-variables patch, I discovered
that the test case I added in commit 11086f2f2 no longer demonstrates
what it's supposed to.  The idea is to tempt the planner to reverse
the order of the two outer joins, which would leave noplace to
correctly evaluate the WHERE clause that's inserted between them.
Before the addition of the delay_upper_joins mechanism, it would
have taken the bait.

However, subsequent improvements broke the test in two different ways.
First, we now recognize the IS NULL coding pattern as an antijoin, and
we won't re-order antijoins; even if we did, the IS NULL test clauses
get removed so there would be no opportunity for them to misbehave.
Second, the planner now discovers that nested parameterized indexscans
are a lot cheaper than the double hash join it used back in the day,
and that approach doesn't want to re-order the joins anyway.  Thus,
in HEAD the test passes even if one dikes out delay_upper_joins.

To fix, change the IS NULL tests to COALESCE clauses, which produce
the same results but the planner isn't smart enough to convert them
to antijoins.  It'll still go for parameterized indexscans though,
so drop the index enabling that (don't know why I added that in the
first place), and disable nestloop joining just to be sure.

This time around, add an EXPLAIN to make the choice of plan visible.

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