os/signal: special-case test settle time on the solaris-amd64-oraclerel builder

This is an attempt to distinguish between a dropped signal and
general builder slowness.

The previous attempt (increasing the settle time to 250ms) still
resulted in a timeout:
https://build.golang.org/log/dd62939f6d3b512fe3e6147074a9c6db1144113f

For #33174

Change-Id: I79027e91ba651f9f889985975f38c7b01d82f634
Reviewed-on: https://go-review.googlesource.com/c/go/+/228266


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