Make test categories composable

Summary:
Previously the add_test_categories would simply overwrite the current set of categories for a
method. This change makes the decorator truly "add" categories, by extending the current set of
categories instead of replacing it.

To do this, I have:
- replaced the getCategories() property on a method (which was itself a method), with a simple
  list property "categories". This makes add_test_categories easier to implement, and test
  categories isn't something which should change between calls anyway.
- rewritten the getCategoriesForTest function to merge method categories with the categories of
  the test case. Previously, it would just use the method categories if they were present. I have
  also greatly simplified this method. Originally, it would use a lot of introspection to enable
  it being called on various types of objects. Based on my tests, it was only ever being called
  on a test case. The new function uses much less introspection then the preivous one, so we
  should easily catch any stray uses, if there are any, as they will generate exceptions now.

Reviewers: zturner, tfiala, tberghammer

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D15451

llvm-svn: 255493