Running tests under valgrind is getting slower at an alarming pace

Hi,

After a recent migration of the skink and a few other animals (sorry for the
false reports on BF, I forgot to adjust a path), I looked at the time it takes
to complete a valgrind run:

9.6: Consumed 4h 53min 18.518s CPU time
10: Consumed 5h 32min 50.839s CPU time
11: Consumed 7h 7min 17.455s CPU time
14: still going at 11h 51min 57.951s
HEAD: 14h 32min 29.571s CPU time

I changed it so that HEAD with be built in parallel separately from the other
branches, so that HEAD gets results within a useful timeframe. But even with
that, the test times are increasing at a rate we're not going to be able to
keep up.

Part of this is caused by a lot of tests running serially, rather than in
parallel. I was pondering setting PROVE_FLAGS=-j5 or something to reduce the
impact of tap tests a bit.

Greetings,

Andres Freund