Breakup TestConcurrentEvents.py into separate test subdirs per test method

This change breaks up the monolithic TestConcurrentEvents.py into a
separate subdir per test method. This allows them to run concurrently,
reduces the chance of a timeout occurring during normal operation, and
allows us to home in on any test methods that may be locking up.

This is step one in the process of squashing timeouts in these test
methods.

The reason for breaking each test method into its own file is to make it
very clear to us if there are a subset of the tests that do in fact lock
up frequently. This will limit how much hunting we need to do to
recreate it.

The reason for putting each file in a separate subdirectory is so that
our concurrent test runner can run multiple test files at the same time.
The unit of serialization in the LLDB test suite is the test directory,
so moving them into separate directories enables the test runner to do
more at the same time.

This change introduces usage of VPATH from gnu make. I use that to
facilitate keeping a single copy of the main.cpp in the parent
concurrent_events directory. Initially I had tried specifying the source
file as ../main.cpp, but our current makefile rules get confused by that
and then also build the output into the parent directory, which defeats
the ability to run each of the test methods concurrently. In the event
that not all systems support VPATH, I can do a bit of surgery on the
Makefile rules and attempt to make it smarter with regards to relative
paths to source files used in the build.

llvm-svn: 276478