test: updated test-stream-pipe-unpipe-stream

test readableStream.unpipe(dest) is no operation when dest is
not a destination for readable stream.

PR-URL: https://github.com/nodejs/node/pull/10100
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

test/parallel/test-stream-pipe-unpipe-streams.js

@@ -25,6 +25,9 @@ source.unpipe(dest2);
 assert.strictEqual(source._readableState.pipes, dest1);
 assert.notStrictEqual(source._readableState.pipes, dest2);
 
+dest2.on('unpipe', common.fail);
+source.unpipe(dest2);
+
 source.unpipe(dest1);
 
 assert.strictEqual(source._readableState.pipes, null);