graph: Add a test close to the tgba_explicit_string setup.

* src/graph/graph.hh (boxed_label): If State_Data==void, inherit from
std::tuple<> and implement a data() method.
(digraph::state_data): Return by reference.
* src/graphtest/ngraph.cc, src/graphtest/ngraph.test: Test the
case where State_Data implements the spot::state interface.

src/graphtest/ngraph.test

@@ -80,7 +80,16 @@ digraph {
  2 [label="s3\n(4,8)"]
  2 -> 1 [label="(4,9)"]
 }
-1 1 1 1 1 1 1 1
+digraph {
+ 0 [label="s1\n"]
+ 0 -> 1 [label="(1,3)"]
+ 0 -> 2 [label="(2,5)"]
+ 1 [label="s2\n"]
+ 1 -> 2 [label="(3,7)"]
+ 2 [label="s3\n"]
+ 2 -> 1 [label="(4,9)"]
+}
+1 1 1 1 1 1 1 1 1
 EOF
 
 diff stdout expected