Add a forwarder handler that alters messages before forwarding, for now,
it just intercepts the "exit" message.
Also, make it easier to write regression tests for a shared gopls
instance, by adding a helper that instantiates two connected
environments, and only runs in the shared execution modes.
Updates golang/go#36879
Updates golang/go#34111
Change-Id: I7673f72ab71b5c7fd6ad65d274c15132a942e06a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/218778
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
