ExpectedWarning doesn't check for warnings on constructors

ExpectedWarning attribute will not check for warnings when on a static constructor. For example, the following test will pass:
RequiresCapability.cs:
```C#
class ClassWithConstructors
{
	[ExpectedWarning ("WarningThatNeverComes", "LoremIpsum")]
	static ClassWithConstructors()
	{
	}

	[ExpectedWarning("NoWarningWillCome", "asdfjkl;")]
	public ClassWithConstructors() { }
}
```