Add analyzer for duplicate test method attribute

[Youssef1313]

The following code will compile successfully:

public class MyTestMethod : TestMethodAttribute
{
    public MyTestMethod([CallerFilePath] string callerFilePath = "", [CallerLineNumber] int callerLineNumber = -1) : base(callerFilePath, callerLineNumber)
    {
    }
}

[TestClass]
public class MyTestClass
{
    [TestMethod]
    [MyTestMethod]
    public void M() { }
}

We should produce a warning so that user picks one of the two attributes.