tests: clear autotools env vars

tests: clear autotools env vars

Fixes automake bug https://bugs.gnu.org/16714.

The testsuite will try and retain these env vars when recursively
running itself, but doesn't distinguish between vars coming from
the env where the tests were launched.  This breaks if the user
happens to `export ACLOCAL=alocal` before `make check`.

This gets a little more confusing in that the Makefile appears to
export these already:
ACLOCAL = ".../automake/pre-inst-env" aclocal-1.16

In reality, while those are set in the make execution environment,
they aren't exported into the process environment, so children
(i.e. shell processes in make rules) don't have them set.  That's
why tests work for most people today.

However, if the user has first exported "ACLOCAL" in the parent
make environment (regardless of value), then make's value will
reset the process environment, and then that will leak into the
children.  That's why we see errors that look like the makefile
env vars are leaking for these people.

At any rate, the fix is to update the test harness to clear these
vars that the test suite relies upon, especially the ones that are
also set in the Makefiles.  That includes AUTOUPDATE even though
it currently isn't used inside any of the tests.

* t/local.mk: Add ACLOCAL, AUTOCONF, AUTOHEADER, AUTOMAKE, and
AUTOUPDATE to the env unset list.
* t/ax/runtest.in: Likewise.