cmd/go: diff .so files quietly in TestScript/build_plugin_reproducible

cmd/go: diff .so files quietly in TestScript/build_plugin_reproducible

This avoids printing verbose binary data and making bell sounds when the
test fails. The binary data can be inspected via other means if needed.

For #58557.

Change-Id: Ia1c4f2c6b9ff2cf6f97611cf335b978fc7bb201f
Reviewed-on: https://go-review.googlesource.com/c/go/+/494577
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>