First, we introduce a flag to teach src/test/test to split its work
into chunks.  Then we replace our invocation of src/test/test in our
"make check" target with a set of 8 scripts that invoke the first
8th of the tests, the second 8th, and so on.

This change makes our "make -kj4 check" target in our hardened
gitlab build more than twice as fast, since src/test/test was taking
the longest to finish.

Closes 40098.

Without this fix, running this test on its own would fail.

Fixes bug 40099. Bugfix on ade50058 in 0.2.8.1-alpha.

"ours" merge to avoid MR 122 -- we already merged MR 123.

For clients, there is no minimum value; in both cases, we warn if
the value seems too low.

Closes ticket 24308.