Add a more generic stepping benchmark, which uses the '-k' option of the test driver

to be able to specify the runhook(s) to bring the debug session to a certain state
before running the benchmarking logic.  An example,

./dotest.py -v -t +b -k 'process attach -n Mail' -k 'thread backtrace all' -p TestRunHooksThenSteppings.py

spawns lldb, attaches to the 'Mail' application, does a backtrace for all threads, and then
runs the benchmark to step the inferior multiple times.

llvm-svn: 141740