test: add regression test for unpipe()

Since 2e568d9 there is a bug where unpiping a stream
from a readable stream that has `_readableState.pipesCount > 1`
will cause it to remove the first stream in the
`_.readableState.pipes` array no matter where in the list the
`dest` stream was.

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

@@ -0,0 +1,26 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+
+const { Readable, Writable } = require('stream');
+
+const source = Readable({read: () => {}});
+const dest1 = Writable({write: () => {}});
+const dest2 = Writable({write: () => {}});
+
+source.pipe(dest1);
+source.pipe(dest2);
+
+dest1.on('unpipe', common.mustCall(() => {}));
+dest2.on('unpipe', common.mustCall(() => {}));
+
+// Should be able to unpipe them in the reverse order that they were piped.
+
+source.unpipe(dest2);
+
+assert.strictEqual(source._readableState.pipes, dest1);
+assert.notStrictEqual(source._readableState.pipes, dest2);
+
+source.unpipe(dest1);
+
+assert.strictEqual(source._readableState.pipes, null);