test: added tests for setEncoding error check

added test to ensure setEncoding inside socket connection
would throw an error

Fixes: nodejs#18118
Ref: nodejs#18178

test/parallel/test-http-socket-encoding-error.js

@@ -0,0 +1,23 @@
+'use strict';
+
+const common = require('../common');
+const http = require('http');
+
+const server = http.createServer().listen(0, connectToServer);
+
+server.on('connection', (socket) => {
+  common.expectsError(() => socket.setEncoding(''),
+                      {
+                        code: 'ERR_METHOD_NOT_IMPLEMENTED',
+                        type: Error
+                      });
+
+  socket.end();
+});
+
+function connectToServer() {
+  const client = new http.Agent().createConnection(this.address().port, () => {
+    client.end();
+  })
+    .on('end', () => server.close());
+}