tests: call the parser from another compilation unit.

In order to improve the testing of %defines, which exports the
interface of the generated parser, change the calc.at tests so that
when %defines is passed, main will be in another compilation unit.  It
loads the generated header.

* tests/calc.at (AT_CALC_MAIN): New.
Includes the definition of the global variables.
Therefore, now declare them from the %requires section of the parser.
Adjust to yydebug and yyparse being renamed by %name-prefix.