singleflight: skip tests involving exec on js/wasm

The js port does not yet support os/exec.

Updates golang/go#37100
Updates golang/go#33519

Change-Id: I9608b7febfdc274dc1b9f34a92d00ef7bea4e13c
Reviewed-on: https://go-review.googlesource.com/c/sync/+/260717
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

singleflight/singleflight_test.go

@@ -253,6 +253,10 @@ func TestGoexitDo(t *testing.T) {
 }
 
 func TestPanicDoChan(t *testing.T) {
+	if runtime.GOOS == "js" {
+		t.Skipf("js does not support exec")
+	}
+
 	if os.Getenv("TEST_PANIC_DOCHAN") != "" {
 		defer func() {
 			recover()
@@ -291,6 +295,10 @@ func TestPanicDoChan(t *testing.T) {
 }
 
 func TestPanicDoSharedByDoChan(t *testing.T) {
+	if runtime.GOOS == "js" {
+		t.Skipf("js does not support exec")
+	}
+
 	if os.Getenv("TEST_PANIC_DOCHAN") != "" {
 		blocked := make(chan struct{})
 		unblock := make(chan struct{})