Fix broken library testcases

This pull request is to fix broken library test cases. They are broken because of multiple reasons:
  1. Null reference exception is thrown out when importing a .ds file which contains a global function. It is because function ``ImportProcedure(string library, ClassNode classNode, ProcedureNode procNode)``` will be called for imported function, and it expects a class node as its second parameter. As a global function doesn't belong to any class, the argument is null. As a procedure node contains its class scope, I remove the second parameter from this function and try to get the corresponding class node internally. 
  2. ``Dynamo.Nodes.Utilities.MakeRelativePath()`` and ``Dynamo.Nodes.Utilities.MakeAbsolutePath()`` are inconsistent. The second one won't create an absolute path for a valid relative path which is without any directory separator like "/" or "\\", e.g., "foo.ds", and the first one may return a relative path without containing any directory separator as well. That said, the path returned from the first function can't be converted to an absolute path by the second one. This cause assembly resolution failure if the assembly is in the same directory as .dyn file. So ".\\" is appended to fix this case.
  3. Related .dyn files are updated.
  4. Invalid test case is deleted. 

@benglin, please help to review the pull request. 
I am not sure if you really mean the second condition ```proc.MethodAttribute != null``` in the following ```if``` statement. I almost certain that you meant something else. But then even when that is corrected, it is still overly confusing. Wouldn't it be nice if we can restructure the who thing to (completely without the ```if (isVisible)``` condition):

```cs
// We will use the same notion for both 'proc' and 'class', so make them both 'is-visible'.
var isProcVisible = ((proc.MethodAttribute == null) || !proc.MethodAttribute.HiddenInLibrary);
...
var isClassVisible = ((classAttributes == null) || !classAttributes.HiddenInLibrary);

// A procedure is only visible if itself is visible, 
// and the class that owns it (if any), is visible.
var finalVisibility = isProcVisible && isClassVisible;
```

Does that make more sense?