net: use /dev/null for TestSplice of big file to fix the test flakes …

…on linux-riscv64-jsing

Fixes #58787

Change-Id: Ia6e9ee0d8dd505f64bfe9ffd23a868b8165f550f
Reviewed-on: https://go-review.googlesource.com/c/go/+/471995
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Andy Pan <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

src/net/splice_test.go

@@ -108,7 +108,7 @@ func (tc spliceTestCase) test(t *testing.T) {
 }
 
 func (tc spliceTestCase) testFile(t *testing.T) {
- f, err := os.CreateTemp(t.TempDir(), "linux-splice-to-file")
+ f, err := os.OpenFile(os.DevNull, os.O_WRONLY, 0)
  if err != nil {
  t.Fatal(err)
  }