Add a "watchpoint" test category and annotate tests appropriately

Most of the watchpoint tests are organized into subtrees, so we can use the
file-based .categories approach to annotate them. The exception are the
concurrent_events tests, which needed to be annotated on a per-test basis.

The motivation behind this is to provide an easy way to disable watchpoint
tests on systems where the watchpoint functionality is not present/unreliable.

llvm-svn: 317004