added test to fs.readlink.spec.js

tests/spec/fs.readlink.spec.js

@@ -32,6 +32,21 @@ describe('fs.readlink', function() {
     });
   });
 
+  it('should return an error if the path is not a symbolic link', function (done) {
+    const fs = util.fs();
+
+    fs.mkdir('/tmp', function (error) {
+      if (error) throw error;
+
+      fs.readlink('/tmp', function (error) {
+        expect(error).to.exist;
+        expect(error.code).to.equal('EINVAL');
+        done();
+      });
+
+    });
+  });
+
   it('should return the contents of a symbolic link', function (done) {
     const fs = util.fs();