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 <bcmills@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>