Fix 'ping6' invocation on MacOS

'ping6' on MacOS does not support a timeout parameter. Here are its options:

```
usage: ping6 [-DdfHmnNoqrRtvwW] [-a addrtype] [-b bufsiz] [-c count]
             [-g gateway] [-h hoplimit] [-I interface] [-i wait] [-l preload]
             [-p pattern] [-S sourceaddr] [-s packetsize] [-z tclass] [-k traffic_class] [-K net_service_type] [hops ...] host
Apple specific options (to be specified before hops or host like all options)
            -b boundif           # bind the socket to the interface
            -k traffic_class     # set traffic class socket option
            -K net_service_type  # set traffic class socket options
            -apple-connect       # call connect(2) in the socket
            -apple-time          # display current time
            -apple-progress      # show progress for debugging
```

Bug: 159463878
Test: run locally on MacOS with IPv6 DUT: act.py -tc Sl4fSanityTest
Change-Id: Id329cda71d5378b67537fd077caa6c5f07391cd5
1 file changed
tree: f4d3c0355826ef5ae01a5781fd06a9810837acc5
  1. acts/
  2. acts_tests/
  3. tools/
  4. wts-acts/
  5. Android.mk
  6. OWNERS
  7. PREUPLOAD.cfg