* tests/calc.at, tests/output.at, tests/regression.at,

* tests/calc.at, tests/output.at, tests/regression.at,
* tests/testsuite.at, tests/torture.at: Rely on Autotest 2.52g:
now the tests are run in private dirs, therefore AC_CLEANUP and
family can be simplified to 0-ary.
* tests/atlocal.in: Now that we run `elsewhere' than in tests/,
use abs. path to find config.h.
* tests/calc.at (AT_CHECK_CALC): Don't try to check the compiler's
stderr, there can be way too much random noise.
Instead pass -Werror to GCC and rely on the exit status.
Reported by Wolfram Wagner.