fix darwin_log test errors on macOS < 10.12
fix darwin_log test errors on macOS < 10.12

The newer event-based tests I added neglected to do the
macOS 10.12 check in the setup.  This caused earlier macOS
test suite runs to attempt to compile code that doesn't exist.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@279672 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb