runtime/pprof: slow new goroutine launches in test

The goroutine profiler tests include one that launches a steady stream
of goroutines. That creates a scheduler busy loop that can prevent
forward progress in the rest of the program. Slow down the launches a
bit so other goroutines have a chance to run.

Fixes #52916
For #52934

Change-Id: I748557201b94918b1fa4960544a51a48d9cacc6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/406654
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: rhysh

src/runtime/pprof/pprof_test.go

@@ -1527,6 +1527,10 @@ func TestGoroutineProfileConcurrency(t *testing.T) {
 			SetGoroutineLabels(WithLabels(ctx, Labels(t.Name()+"-churn-i", fmt.Sprint(i))))
 			if i == 0 {
 				ready.Done()
+			} else if i%16 == 0 {
+				// Yield on occasion so this sequence of goroutine launches
+				// doesn't monopolize a P. See issue #52934.
+				runtime.Gosched()
 			}
 			if ctx.Err() == nil {
 				go churn(i + 1)