PDL: tell ‘diff’ to output a colored diff

This makes it much easier to see the added and removed lines in the
logs. Use ‘less -R’ when viewing a log file to allow the escape
sequences to show up on the terminal.

Test: a provoked error shows a colorful diff: http://screen/bEf5puAqZGVbC5w
Change-Id: I3318015ec563e47be5317298464ef527d1809424

tools/pdl/src/test_utils.rs

@@ -73,6 +73,7 @@ pub fn diff(left: &str, right: &str) -> String {
     // We expect `diff` to be available on PATH.
     let output = Command::new("diff")
         .arg("--unified")
+        .arg("--color=always")
         .arg("--label")
         .arg("left")
         .arg("--label")