internal/lsp/lsprpc: update binder tests to handle forwarding

Update the new binder tests to run both with a standalone server, and
with a forwarding chain.

Make a few superficial improvements along the way as well.

Change-Id: Icd197698093a3f6149ab58171806b2388ed75b7f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/321134
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

internal/jsonrpc2/jsonrpc2.go

@@ -57,11 +57,11 @@ func (f HandlerFunc) Handle(ctx context.Context, req *Request) (interface{}, err
 	return f(ctx, req)
 }
 
-// async is a small helper for things with an asynchronous result that you can
-// wait for.
+// async is a small helper for operations with an asynchronous result that you
+// can wait for.
 type async struct {
-	ready  chan struct{}
-	errBox chan error
+	ready  chan struct{} // signals that the operation has completed
+	errBox chan error    // guards the operation result
 }
 
 func newAsync() *async {