tests: restore the tests on -Werror

When run as /bin/sh, Bash sets the shell variable POSIXLY_CORRECT to
y.  The test suite checks for the envvar POSIXLY_CORRECT to turn of
some tests not supported in POSIX mode.  Restore these tests.

Reported by the Hydra build farm, from Rob Vermaas.

* tests/local.at (AT_BISON_CHECK_WARNINGS_): Check the envvar
POSIXLY_CORRECT, not the shell variable.