autoconf: use include directives instead of recursing down tests (mostly)

Only down to tests/zfs-tests/tests, but pull out C programs into the
main Makefile ‒ this means we get correct dependency tracking for all
programs (and parallelise across them)

dist diff:

-zfs-2.1.99/tests/zfs-tests/tests/stress/
-zfs-2.1.99/tests/zfs-tests/tests/stress/Makefile.am
-zfs-2.1.99/tests/zfs-tests/tests/stress/Makefile.in

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13316