Autotest: --output

[Akim Demaille]

Hi!

The Bison test suite, Autotest based, can be run with various
flags to test different environments (e.g., run the whole suite
with a C++ compiler instead of a C compiler to make run it works
properly too, or run the test suite with POSIXLY_CORRECT=1, etc.).

It would be nice to be able to run multiple instances of the same
Autotest-generated testsuite, but to provide some --output or whatever
to replace the hard coded basename based on $0, so that they don't
all use the same "testsuite.log" and "testsuite.dir" names.

Cheers!