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 <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>

Author: dmitshur

src/cmd/go/testdata/script/build_plugin_reproducible.txt

@@ -3,7 +3,7 @@
 
 go build -trimpath -buildvcs=false -buildmode=plugin -o a.so main.go
 go build -trimpath -buildvcs=false -buildmode=plugin -o b.so main.go
-cmp a.so b.so
+cmp -q a.so b.so
 
 -- main.go --
 package main