Change output of compiletest to include the compare-mode when present.

E.g. when running with `--compare-mode=nll`, then each test line will
look like e.g.:

```
test [ui (nll)] ui/issue-10969.rs ... ok
```

Author: pnkfelix

src/tools/compiletest/src/common.rs

@@ -101,7 +101,7 @@ pub enum CompareMode {
 }
 
 impl CompareMode {
-    fn to_str(&self) -> &'static str {
+    pub(crate) fn to_str(&self) -> &'static str {
         match *self {
             CompareMode::Nll => "nll"
         }

src/tools/compiletest/src/main.rs

@@ -728,7 +728,11 @@ pub fn make_test_name(config: &Config, testpaths: &TestPaths) -> test::TestName
     let path = PathBuf::from(config.src_base.file_name().unwrap())
         .join(&testpaths.relative_dir)
         .join(&testpaths.file.file_name().unwrap());
-    test::DynTestName(format!("[{}] {}", config.mode, path.display()))
+    let mode_suffix = match config.compare_mode {
+        Some(ref mode) => format!(" ({})", mode.to_str()),
+        None => format!(""),
+    };
+    test::DynTestName(format!("[{}{}] {}", config.mode, mode_suffix, path.display()))
 }
 
 pub fn make_test_closure(config: &Config, testpaths: &TestPaths) -> test::TestFn {