Parallelize src/test/test into chunks.

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.

changes/parallel_unit_test

@@ -0,0 +1,4 @@
+  o Minor features (tests):
+    - Our "make check" target now runs the unit tests in 8 parallel chunks.
+      Doing this speeds up hardened CI builds by more than a factor of two.
+      Closes ticket 40098.