os: fix Chtimes test flakes

It appears that some builders (notably, linux-arm) have some additional
security software installed, which apparently reads the files created by
tests. As a result, test file atime is changed, making the test fail
like these:

=== RUN   TestChtimesOmit
    ...
    os_test.go:1475: atime mismatch, got: "2024-07-30 18:42:03.450932494 +0000 UTC", want: "2024-07-30 18:42:02.450932494 +0000 UTC"

=== RUN   TestChtimes
    ...
    os_test.go:1539: AccessTime didn't go backwards; was=2024-07-31 20:45:53.390326147 +0000 UTC, after=2024-07-31 20:45:53.394326118 +0000 UTC

According to inode(7), atime is changed when more than 0 bytes are read
from the file. So, one possible solution to these flakes is to make the
test files empty, so no one can read more than 0 bytes from them.

Fixes #68687
Fixes #68663

Change-Id: Ib9234567883ef7b16ff8811e3360cd26c2d6bdab
Reviewed-on: https://go-review.googlesource.com/c/go/+/604315
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Kirill Kolyshkin <kolyshkin@gmail.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

Author: kolyshkin

src/os/os_test.go

@@ -1375,8 +1375,7 @@ func TestChtimes(t *testing.T) {
 	t.Parallel()
 
 	f := newFile(t)
-
-	f.Write([]byte("hello, world\n"))
+	// This should be an empty file (see #68687, #68663).
 	f.Close()
 
 	testChtimes(t, f.Name())
@@ -1394,12 +1393,9 @@ func TestChtimesOmit(t *testing.T) {
 func testChtimesOmit(t *testing.T, omitAt, omitMt bool) {
 	t.Logf("omit atime: %v, mtime: %v", omitAt, omitMt)
 	file := newFile(t)
-	_, err := file.Write([]byte("hello, world\n"))
-	if err != nil {
-		t.Fatal(err)
-	}
+	// This should be an empty file (see #68687, #68663).
 	name := file.Name()
-	err = file.Close()
+	err := file.Close()
 	if err != nil {
 		t.Error(err)
 	}