Add unit test for Link label

Add unit tests for PR#10642

Using the following code in a .net core project reproduces issue #10515,

• When the linkLabel.Text is null or empty, the code Application.DoEvents(); will enter an infinite loop before PR#10642

 LinkLabel linkLabel = new();
 // try to comment out  this line 'linkLabel.Text = "2";'
 // and observe the result after click the button
 linkLabel.Text = "";

this.Controls.Add(linkLabel);
this.Invalidate();
Application.DoEvents(); 

MessageBox.Show("button1_Click");

However, Application.DoEvents(); is invalid when used in Unit test, so additional tests need to be added for PR#https://github.com/dotnet/winforms/pull/10642