autoconf: use include directives instead of recursing down test data

We drop /multiple/ seconds off the generation, a dozen off a clean
rebuild, 185 files, and trivialise the distribution,
which can now be trivially generated via the provided snippets

Dist diff:

-zfs-2.1.99/tests/zfs-tests/tests/functional/pam/utilities.kshlib
+zfs-2.1.99/tests/zfs-tests/tests/functional/pam/utilities.kshlib.in

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