add availability check to DarwinLog event tests

The pexpect-based tests properly checked for the stub reporting
DarwinLog support.  The event-based ones did not.  This is fixed
here.  Swift CI bots are not currently building debugserver on
macOS, so they don't have the DarwinLog support even when they
pass the macOS 10.12 check.

llvm-svn: 281696