testing: add PAUSE, CONT output lines to explain Parallel execution

This should make parallel execution a bit clearer.
With -p=1 it should make the execution completely unambiguous.

Fixes #19280.

Change-Id: Ib48cdfe96896d01b0d8f98ccb2fab614407a7d92
Reviewed-on: https://go-review.googlesource.com/49430
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/testing/testing.go

@@ -675,9 +675,30 @@ func (t *T) Parallel() {
 	t.parent.sub = append(t.parent.sub, t)
 	t.raceErrors += race.Errors()
 
+	if t.chatty {
+		// Print directly to root's io.Writer so there is no delay.
+		root := t.parent
+		for ; root.parent != nil; root = root.parent {
+		}
+		root.mu.Lock()
+		fmt.Fprintf(root.w, "=== PAUSE %s\n", t.name)
+		root.mu.Unlock()
+	}
+
 	t.signal <- true   // Release calling test.
 	<-t.parent.barrier // Wait for the parent test to complete.
 	t.context.waitParallel()
+
+	if t.chatty {
+		// Print directly to root's io.Writer so there is no delay.
+		root := t.parent
+		for ; root.parent != nil; root = root.parent {
+		}
+		root.mu.Lock()
+		fmt.Fprintf(root.w, "=== CONT  %s\n", t.name)
+		root.mu.Unlock()
+	}
+
 	t.start = time.Now()
 	t.raceErrors += -race.Errors()
 }