Don't require the output from libtest to be valid UTF-8

On Windows this is sometimes not the case, for reasons I can't track down.

This works around the problem, although I'm not sure how to confirm we're not generating invalid
build metrics in this case.

Author: jyn514

src/bootstrap/render_tests.rs

@@ -88,10 +88,10 @@ impl<'a> Renderer<'a> {
     }
 
     fn render_all(mut self) {
-        let mut line = String::new();
+        let mut line = Vec::new();
         loop {
             line.clear();
-            match self.stdout.read_line(&mut line) {
+            match self.stdout.read_until(b'\n', &mut line) {
                 Ok(_) => {}
                 Err(err) if err.kind() == std::io::ErrorKind::UnexpectedEof => break,
                 Err(err) => panic!("failed to read output of test runner: {err}"),
@@ -100,12 +100,13 @@ impl<'a> Renderer<'a> {
                 break;
             }
 
-            match serde_json::from_str(&line) {
+            match serde_json::from_slice(&line) {
                 Ok(parsed) => self.render_message(parsed),
                 Err(_err) => {
                     // Handle non-JSON output, for example when --nocapture is passed.
-                    print!("{line}");
-                    let _ = std::io::stdout().flush();
+                    let mut stdout = std::io::stdout();
+                    stdout.write_all(&line).unwrap();
+                    let _ = stdout.flush();
                 }
             }
         }