tests: port AT_CHECK_ENV to hosts with flaky grep

tests: port AT_CHECK_ENV to hosts with flaky grep

* tests/local.at (AT_CHECK_ENV): Don't assume that if one grep
fails, the other will too.  It could be that 'grep' is flaky,
and fails somewhat at random.  This would explain the problems
reported for autoconf-2.68b on FreeBSD and MacOS X, for example:
<http://lists.gnu.org/archive/html/bug-autoconf/2012-03/msg00032.html>
<http://lists.gnu.org/archive/html/bug-autoconf/2012-03/msg00035.html>
<http://lists.gnu.org/archive/html/bug-autoconf/2012-03/msg00036.html>
<http://lists.gnu.org/archive/html/bug-autoconf/2012-03/msg00044.html>