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_test.go

@@ -126,7 +126,7 @@ func TestConnectionHeader(t *testing.T) {
 func testConnection(t *testing.T, framer jsonrpc2.Framer) {
 	stacktest.NoLeak(t)
 	ctx := eventtest.NewContext(context.Background(), t)
-	listener, err := jsonrpc2.NetPipe(ctx)
+	listener, err := jsonrpc2.NetPipeListener(ctx)
 	if err != nil {
 		t.Fatal(err)
 	}