Add timeout to udp multicast join tests

[squeek502]

This is one way to close #539. See #539 (comment) for a bit more context.

With the default settings of firewalld (used by Fedora, RHEL, etc), incoming messages from multicast IPs are dropped, meaning that the multicast test will hang forever. This PR introduces a 1 second timeout, after which the test will fail.

When the tests timeout, it'll look something like:

  timeout (could be caused by firewall settings)
  .\lib/tap.lua:66: Found 1 unexpected call
  stack traceback:
        [C]: in function 'error'
        .\lib/tap.lua:66: in function <.\lib/tap.lua:48>
        [C]: in function 'xpcall'
        .\lib/tap.lua:48: in function 'run'
        .\lib/tap.lua:146: in function <.\lib/tap.lua:122>
        [C]: at 0x7ff7494359f0
not ok 6 udp multicast join ipv4
  timeout (could be caused by firewall settings)
  .\lib/tap.lua:66: Found 1 unexpected call
  stack traceback:
        [C]: in function 'error'
        .\lib/tap.lua:66: in function <.\lib/tap.lua:48>
        [C]: in function 'xpcall'
        .\lib/tap.lua:48: in function 'run'
        .\lib/tap.lua:146: in function <.\lib/tap.lua:122>
        [C]: at 0x7ff7494359f0
not ok 7 udp multicast join ipv6