os: deflake TestFdReadRace

The test would hang if the call to Fd set the pipe to be non-blocking
before the Read entered the first read system call. Avoid that problem
by writing data to the pipe to wake up the read.

For #24481
Fixes #44818

Change-Id: I0b798874c7b81e7308a38ebbf657efc4392ffacd
Reviewed-on: https://go-review.googlesource.com/c/go/+/322893


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>