Testing "workers launched" in expected output? Really?

So buildfarm member piculet just fell over like this:

================== pgsql.build/src/test/regress/regression.diffs ==================
*** /home/andres/build/buildfarm-piculet/HEAD/pgsql.build/../pgsql/src/test/regress/expected/select_parallel.out 2018-02-28 16:10:01.986941733 +0000
--- /home/andres/build/buildfarm-piculet/HEAD/pgsql.build/src/test/regress/results/select_parallel.out 2018-03-02 19:13:57.843939790 +0000
***************
*** 485,495 ****
QUERY PLAN
--------------------------------------------------------------------------
Aggregate (actual rows=1 loops=1)
! -> Nested Loop (actual rows=98000 loops=1)
-> Seq Scan on tenk2 (actual rows=10 loops=1)
Filter: (thousand = 0)
Rows Removed by Filter: 9990
! -> Gather (actual rows=9800 loops=10)
Workers Planned: 4
Workers Launched: 4
-> Parallel Seq Scan on tenk1 (actual rows=1960 loops=50)
--- 485,495 ----
QUERY PLAN
--------------------------------------------------------------------------
Aggregate (actual rows=1 loops=1)
! -> Nested Loop (actual rows=97836 loops=1)
-> Seq Scan on tenk2 (actual rows=10 loops=1)
Filter: (thousand = 0)
Rows Removed by Filter: 9990
! -> Gather (actual rows=9784 loops=10)
Workers Planned: 4
Workers Launched: 4
-> Parallel Seq Scan on tenk1 (actual rows=1960 loops=50)

======================================================================

and now I am on the warpath. I have no idea whether or not the diff
here is significant --- maybe it is --- but I am desperately unhappy
that we have expected-output files that will fail if fewer than the
expected number of workers launched. I find that absolutely
unacceptable. It reminds me entirely too much of when I had to package
MySQL for Red Hat, and half the time the package builds failed in
Red Hat's buildfarm, because their tests weren't robust about passing
on heavily loaded machines. I won't stand for our tests becoming
like that.

Perhaps we could deal with this by suppressing the Workers Planned/
Launched lines when we are suppressing costs?

regards, tom lane