Unit testing for def/ module.

Added unit tests to test code in def/ module.

vendor/golang.org/x/sys/unix/fcntl_darwin.go

@@ -0,0 +1,18 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+import "unsafe"
+
+// FcntlInt performs a fcntl syscall on fd with the provided command and argument.
+func FcntlInt(fd uintptr, cmd, arg int) (int, error) {
+	return fcntl(int(fd), cmd, arg)
+}
+
+// FcntlFlock performs a fcntl syscall for the F_GETLK, F_SETLK or F_SETLKW command.
+func FcntlFlock(fd uintptr, cmd int, lk *Flock_t) error {
+	_, err := fcntl(int(fd), cmd, int(uintptr(unsafe.Pointer(lk))))
+	return err
+}