Build testsuite with newer autoconf.

Build testsuite with newer autoconf.

* tests/output.at (m4_expand): Don't override in newer autoconf,
where the underlying implementation changed.
* tests/cxx-type.at (_AT_RESOLVED_GLR_OUTPUT)
(_AT_RESOLVED_GLR_OUTPUT_WITH_LOC, _AT_AMBIG_GLR_OUTPUT)
(_AT_AMBIG_GLR_OUTPUT_WITH_LOC, _AT_GLR_STDERR)
(_AT_VERBOSE_GLR_STDERR): Expand to double-quoted strings,
since some of them contain unbalanced ')'.

Signed-off-by: Eric Blake <ebb9@byu.net>