test: update test case in test-net-internal.js

Add test code for `makeSyncWrite`, which improve
`test/parallel/test-net-internal.js` test coverage to 100%

test/parallel/test-net-internal.js

@@ -2,9 +2,9 @@
 
 // Flags: --expose-internals
 
-require('../common');
+const common = require('../common');
 const assert = require('assert');
-const isLegalPort = require('internal/net').isLegalPort;
+const { isLegalPort, makeSyncWrite } = require('internal/net');
 
 for (let n = 0; n <= 0xFFFF; n++) {
   assert(isLegalPort(n));
@@ -18,3 +18,38 @@ const bad = [-1, 'a', {}, [], false, true, 0xFFFF + 1, Infinity,
              -Infinity, NaN, undefined, null, '', ' ', 1.1, '0x',
              '-0x1', '-0o1', '-0b1', '0o', '0b'];
 bad.forEach((i) => assert(!isLegalPort(i)));
+
+
+function writeToStdout() {
+  const ctx = {
+    _handle: {
+      bytesWritten: 0
+    }
+  };
+  const syncWrite = makeSyncWrite(process.stdout.fd);
+  syncWrite.call(ctx, 'hello', 'utf-8', common.mustCall((ex) => {
+    assert.strictEqual(undefined, ex);
+  }));
+  syncWrite.call(ctx, Buffer.from('world'), 'buffer', common.mustCall((ex) => {
+    assert.strictEqual(undefined, ex);
+  }));
+  assert.strictEqual(ctx._handle.bytesWritten, 10);
+}
+function writeToInvalidFD() {
+  const ctx = {
+    _handle: {
+      bytesWritten: 0
+    }
+  };
+  const invalidFD = -1;
+  const syncWrite = makeSyncWrite(invalidFD);
+  syncWrite.call(ctx, Buffer.from('world'), 'buffer', common.mustCall((ex) => {
+    assert.strictEqual(ex.code, 'EBADF');
+  }));
+  assert.strictEqual(ctx._handle.bytesWritten, 5);
+}
+if (!common.isWindows) {
+  // This test will crash on windows, so skip it
+  writeToStdout();
+  writeToInvalidFD();
+}