os: deflake TestFdReadRace by increasing timeout

This timeout will never be reached if the test passes, so it doesn't
much matter how long it is. The test is t.Parallel so on a slow system
1 second may occasionally not be enough, although on my laptop the
test takes about 0.02 seconds.

Fixes #34431

Change-Id: Ia2184e6be3747933bfe83aa6c8e1f77e6b1e0bba
Reviewed-on: https://go-review.googlesource.com/c/go/+/200764


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>