os/signal: scale back the solaris-amd64-oraclerel settle time

os/signal: scale back the solaris-amd64-oraclerel settle time

The settleTime is arbitrary. Ideally we should refactor the test to
avoid it (using subprocesses instead of sleeps to isolate tests from
each others' delayed signals), but as a shorter-term workaround let's
try scaling it back to match linux/ppc64 (the other builder that
empirically requires a longer settleTime).

For #51054.
Updates #33174.

Change-Id: I574fffaadd74c52c13d63974e87f20b6d3cf3c4b
Reviewed-on: https://go-review.googlesource.com/c/go/+/403199
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>