remove leading whitespace from single-line test snapshot

Author: narpfel

src/write.rs

@@ -1044,9 +1044,7 @@ mod tests {
                 .finish()
                 .write_to_string(Source::from("")),
         );
-        assert_snapshot!(msg, @r###"
-        Error: can't compare apples with oranges
-        "###)
+        assert_snapshot!(msg, @r###"Error: can't compare apples with oranges"###)
     }
 
     #[test]