tests: Fix dot test by not including whitespace.

Rust 1.35 changed the behavior of the Debug trait to include the
trailing comma in structs and other things. This deals with that
by not including all of the whitespace.

The Rust change that introduced this was rust-lang/rust#59076

Fixes petgraph#255.

tests/graph.rs

@@ -1422,12 +1422,12 @@ fn dot() {
     let mut gr = Graph::new();
     let a = gr.add_node(Record { a: 1, b: r"abc\" });
     gr.add_edge(a, a, (1, 2));
-    let dot_output = format!("{:#?}", Dot::new(&gr));
+    let dot_output = format!("{:?}", Dot::new(&gr));
     assert_eq!(dot_output,
     // The single \ turns into four \\\\ because of Debug which turns it to \\ and then escaping each \ to \\.
 r#"digraph {
-    0 [label="Record {\l    a: 1,\l    b: \"abc\\\\\"\l}\l"]
-    0 -> 0 [label="(\l    1,\l    2\l)\l"]
+    0 [label="Record { a: 1, b: \"abc\\\\\" }"]
+    0 -> 0 [label="(1, 2)"]
 }
 "#);
 }