Also test parse error messages, including with YYERROR_VERBOSE.

* tests/calc.m4 (calc.y): Add support for `exp = exp' (non
associative).
Use it to check the computations.
Use it to check `nonassoc' is honored.
(AT_DATA_CALC_Y): Equip `calc.y' with YYERROR_VERBOSE when passed
`--yyerror-verbose'.
(_AT_CHECK_CALC): Adjust to this option.
(_AT_CHECK_CALC_ERROR): New macro to check parse error messages.