test: stdin is not always a net.Socket

`<`-ing a file into stdin actually results in a `fs.ReadStream`, rather
than a `tty.ReadStream`, and as such does not inherit from net.Socket,
unlike the other possible stdin options.

Refs: #5916
PR-URL: #5935
Reviewed-By: Colin Ihrig <[email protected]>

test/known_issues/test-stdin-is-always-net.socket.js

@@ -0,0 +1,19 @@
+'use strict';
+// Refs: https://github.com/nodejs/node/pull/5916
+
+const common = require('../common');
+const assert = require('assert');
+const spawn = require('child_process').spawn;
+const net = require('net');
+
+if (process.argv[2] === 'child') {
+  assert(process.stdin instanceof net.Socket);
+  return;
+}
+
+const proc = spawn(process.execPath, [__filename, 'child'], { stdio: 'ignore' });
+// To double-check this test, set stdio to 'pipe' and uncomment the line below.
+// proc.stderr.pipe(process.stderr);
+proc.on('exit', common.mustCall(function(exitCode) {
+  process.exitCode = exitCode;
+}));