dot: name the digraph

* spot/twaalgos/dot.cc: Here.
* NEWS: Mention the change.
* tests/core/alternating.test, tests/core/det.test,
tests/core/dstar.test, tests/core/monitor.test,
tests/core/neverclaimread.test, tests/core/readsave.test,
tests/core/sccdot.test, tests/core/tgbagraph.test,
tests/python/_altscc.ipynb, tests/python/_autparserr.ipynb,
tests/python/alternation.ipynb, tests/python/atva16-fig2a.ipynb,
tests/python/atva16-fig2b.ipynb, tests/python/automata-io.ipynb,
tests/python/automata.ipynb, tests/python/decompose.ipynb,
tests/python/gen.ipynb, tests/python/highlighting.ipynb,
tests/python/ltsmin-dve.ipynb, tests/python/ltsmin-pml.ipynb,
tests/python/parity.ipynb, tests/python/product.ipynb,
tests/python/randaut.ipynb, tests/python/satmin.ipynb,
tests/python/stutter-inv.ipynb, tests/python/testingaut.ipynb,
tests/python/word.ipynb: Adjust test cases.

tests/core/neverclaimread.test

@@ -132,7 +132,7 @@ EOF
 run 0 ../ikwiad -XN input > stdout
 
 cat >expected <<EOF
-digraph G {
+digraph "" {
   rankdir=LR
   label="\n[Büchi]"
   labelloc="t"
@@ -327,7 +327,7 @@ accept_all:
 EOF
 autfilt --name=%F --dot=nsc <input >stdout 2>stderr && exit 1
 cat >expected <<EOF
-digraph G {
+digraph "-" {
   rankdir=LR
   label="-\n[Büchi]"
   labelloc="t"