os: skip Root.Chtimes atime check on netbsd, truncate a/mtime on plan9

The NetBSD builder has noatime set on its filesystem.
Skip testing the atime on this builder.

Plan9 has second precision on its atime and mtimes.
Truncate the values passed to Chtimes.

For #72957

Change-Id: I963e2dd34075a9ba025e80641f0b675d5d912188
Reviewed-on: https://go-review.googlesource.com/c/go/+/659356
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/os/os_test.go

@@ -1345,6 +1345,9 @@ var hasNoatime = sync.OnceValue(func() bool {
 	// but the syscall is OS-specific and is not even wired into Go stdlib.
 	//
 	// Only used on NetBSD (which ignores explicit atime updates with noatime).
+	if runtime.GOOS != "netbsd" {
+		return false
+	}
 	mounts, _ := ReadFile("/proc/mounts")
 	return bytes.Contains(mounts, []byte("noatime"))
 })

src/os/root_test.go

@@ -449,7 +449,8 @@ func TestRootChtimes(t *testing.T) {
 				atime: time.Now(),
 				mtime: time.Time{},
 			}} {
-				if runtime.GOOS == "js" {
+				switch runtime.GOOS {
+				case "js", "plan9":
 					times.atime = times.atime.Truncate(1 * time.Second)
 					times.mtime = times.mtime.Truncate(1 * time.Second)
 				}
@@ -465,8 +466,10 @@ func TestRootChtimes(t *testing.T) {
 				if got := st.ModTime(); !times.mtime.IsZero() && !got.Equal(times.mtime) {
 					t.Errorf("after root.Chtimes(%q, %v, %v): got mtime=%v, want %v", test.open, times.atime, times.mtime, got, times.mtime)
 				}
-				if got := os.Atime(st); !times.atime.IsZero() && !got.Equal(times.atime) {
-					t.Errorf("after root.Chtimes(%q, %v, %v): got atime=%v, want %v", test.open, times.atime, times.mtime, got, times.atime)
+				if !hasNoatime() {
+					if got := os.Atime(st); !times.atime.IsZero() && !got.Equal(times.atime) {
+						t.Errorf("after root.Chtimes(%q, %v, %v): got atime=%v, want %v", test.open, times.atime, times.mtime, got, times.atime)
+					}
 				}
 			}
 		})