os: force a goroutine to be scheduled on WASM

The TestRootConcurrentClose test can fail when GOARCH=WASM because of
goroutine starvation. The spawned goroutine will sometimes run in a
loop and never have the main goroutine be scheduled. This causes the
test to fail due to a timeout. This change forces the goroutine to be
scheduled with each iteration of the loop when  GOARCH=WASM.

For #71134
Fixes #71117

Change-Id: I4fb68907c9ac3b33bd0572d5e5db2974a3379191
Reviewed-on: https://go-review.googlesource.com/c/go/+/640195
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Carlos Amedee <carlos@golang.org>

src/os/root_test.go

@@ -1077,6 +1077,10 @@ func TestRootConcurrentClose(t *testing.T) {
 				first = false
 			}
 			f.Close()
+			if runtime.GOARCH == "wasm" {
+				// TODO(go.dev/issue/71134) can lead to goroutine starvation.
+				runtime.Gosched()
+			}
 		}
 	}()
 	if err := <-ch; err != nil {