We were using file descriptor 100, which requires the Linux kernel to
grow the fdtable size. That step may sometimes require a long time,
causing the test to fail. Switch to file descriptor 30, which should
not require growing the fdtable.

Fixes #23784

Change-Id: I3ac40d6f8569c70d34b470cfca34eff149bf8229
Reviewed-on: https://go-review.googlesource.com/108537
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>