Tune local.at so that people can "autom4te -l autotest calc.at -o

Tune local.at so that people can "autom4te -l autotest calc.at -o
calc" for instance, to extract a sub test suite.
* tests/testsuite.at: Move the initialization, Autotest version
requirement, and AT_TESTED invocation into...
* tests/local.at: here.
* tests/testsuite.at: Include it for compatibility with Autoconf
2.57.
* tests/Makefile.am ($(TESTSUITE)): Report that the warning should
be ignore.