Some M4 cleanup in the testsuite.

Some M4 cleanup in the testsuite.

Suggested by Eric Blake at
<http://lists.gnu.org/archive/html/bison-patches/2009-04/msg00083.html>.
* tests/existing.at (_AT_TEST_EXISTING_GRAMMAR): Do not
complicate the code by distinguishing between a missing value
and an empty string value for an optional argument.  This fix is
allowed by the similar fix in AT_TEST_TABLES_AND_PARSE below.
* tests/local.at (_AT_TEST_TABLES_AND_PARSE): Merge into...
(AT_TEST_TABLES_AND_PARSE): ... this now that the special
arguments are not needed because of the following changes.
Fix stale comments.
Bison developers should use GNU M4 and should not use
POSIXLY_CORRECT when building the test suite, so do not
complicate the code by avoiding $10 and above.
Do not quote an empty string value for an optional argument, and
do not distinguish between a missing value and an empty string
value.