cmd/go: in TestTerminalPassthrough, delay subprocess exit until the PTY has been...

cmd/go: in TestTerminalPassthrough, delay subprocess exit until the PTY has been read

Empirically, unread PTY output may be discarded on macOS when the
child process exits.

Fixes #57141.

Tested with 'go test cmd/go -run=TestTerminalPassthrough -count=1000'
on a darwin-amd64-12_0 gomote.

Change-Id: I11508e6429c61488f30e10d9ae0cc94fdf059257
Reviewed-on: https://go-review.googlesource.com/c/go/+/455915
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>