Add '-e' and '-x' options to the test driver to be able to specify an executable (full path)

and the breakpoint specification for the benchmark purpose.  This is used by TestSteppingSpeed.py
to benchmark the lldb stepping speed.  Without '-e' and 'x' specified, the test defaults to
run the built lldb against itself and stopped on Driver::MainLoop, then stepping for 50 times.

rdar://problem/7511193

llvm-svn: 141584