Allow test categories to be skipped.

This adds a new parameter, --skip-category, that can be used to list
categories that should be skipped. For example, to run all tests except for
Objective-C ones, one can now write:

  ./dotest.py --skip-category objc [...]

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@187590 91177308-0d34-0410-b5e6-96231b3b80d8