test: test the buffered pipe used by nsd (#12563)

Nomad Service Discovery uses an in-memory buffered pipe implementation
to connect consul-template to the Nomad API.

This adds a basic test for that helper functionality.

helper/bufconndialer/bufconndialer_test.go

@@ -0,0 +1,56 @@
+package bufconndialer
+
+import (
+	"io"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestBufConnDialer asserts that bufconndialer creates a dialer connected to a
+// listener.
+func TestBufConnDialer(t *testing.T) {
+	listener, dialer := New()
+
+	cleanup := make(chan struct{})
+	go func() {
+		defer close(cleanup)
+		for {
+			conn, err := listener.Accept()
+			if err != nil {
+				// google.golang.org/grpc/test/bufconn.Listener doesn't
+				// return a net.ErrClosed so we have to compare strings
+				if err.Error() == "closed" {
+					return
+				}
+
+				t.Errorf("error accepting connection: %v", err)
+				return
+			}
+
+			n, err := conn.Write([]byte("ok"))
+			if err != nil {
+				t.Errorf("error writing to connection after %d bytes: %v", n, err)
+				return
+			}
+			if err := conn.Close(); err != nil {
+				t.Errorf("error closing connection: %v", err)
+				return
+			}
+		}
+	}()
+
+	conn, err := dialer.Dial("anything", "goes")
+	require.NoError(t, err)
+
+	buf := make([]byte, 2)
+	_, err = conn.Read(buf)
+	require.NoError(t, err)
+	require.Equal(t, "ok", string(buf))
+
+	_, err = conn.Read(buf)
+	require.EqualError(t, err, io.EOF.Error())
+
+	listener.Close()
+	<-cleanup
+}