Reduce the number of simultaneous debug sessions to 10 and remove

the expectedFlakeyDarwin annotation.

I've been running this test in isolation on my macOS Sierra system
and haven't seen a failure in 20-30 runs.  The number of simultaneous
debug sessions that it spins up could be a problem when the testbots
are running under load, so I'm reducing this from 20 simultaneous
debug sessions to see if we can get enough stability to leave this
enabled.

llvm-svn: 281291