std::cerr will flush after each operator<< by default, so it's simpler
to use \n instead of std::endl, especially if we can merge \n into the
previous string.  Ideally we should prefer \n for std::cout as well,
but there are reasonable cases where we want to call std::endl there,
so it's hard to enforce.

* tests/sanity/style.test: Diagnose occurrences of cerr.*<<.*endl.
* bin/autcross.cc, bin/autfilt.cc, bin/ltlcross.cc, bin/ltlsynt.cc,
spot/tl/formula.cc, spot/twa/bdddict.cc, tests/core/checkpsl.cc,
tests/core/checkta.cc, tests/core/consterm.cc, tests/core/emptchk.cc,
tests/core/equalsf.cc, tests/core/ikwiad.cc, tests/core/kind.cc,
tests/core/length.cc, tests/core/ltlrel.cc, tests/core/parity.cc,
tests/core/randtgba.cc, tests/core/reduc.cc, tests/core/syntimpl.cc,
tests/ltsmin/modelcheck.cc: Fix them.