Add helper to work around Solaris /bin/sh bug in Autotest.

Add helper to work around Solaris /bin/sh bug in Autotest.

This hackjob edits the definition of AT_INIT to fix the problem
described in this patch[1], even for versions of Autoconf which
do not include this fix (which, as of this writing, is all of
them).

Specifically, older Solaris /bin/sh does not correctly handle
redirections on : within a shell function, but Autotest test
suites expect this to work when preparing the environment
for AT_CHECK.

[1] https://lists.gnu.org/archive/html/autoconf-patches/2021-03/msg00000.html