tests use cross-spawn-async instead of child_process.spawn

[josephfrazier]

This allows Windows to run the tests, as promised in
#429 (comment)

Note that there are a couple of outstanding failures on Windows:

# client.seed: filesystem path to folder with one file, string
not ok 8 should be equal
  ---
    operator: equal
    expected: '3a686c32404af0a66913dd5f8d2b40673f8d4490'
    actual:   'e6887b78d89d995876c7cef3476b6fb32c4c4e3d'
  ...
not ok 9 should be equal
  ---
    operator: equal
    expected: |-
      'magnet:?xt=urn:btih:3a686c32404af0a66913dd5f8d2b40673f8d4490&dn=folder&tr=udp%3A%2F%2Ftracker.webtorrent.io%3A80'
    actual: |-
      'magnet:?xt=urn:btih:e6887b78d89d995876c7cef3476b6fb32c4c4e3d&dn=folder&tr=udp%3A%2F%2Ftracker.webtorrent.io%3A80'

[josephfrazier]

Note that there are a couple of outstanding failures on Windows:

Hmm, this appears to have been due to a bad core.autocrlf setting I had in the VM. core.autocrlf was set to true, so ./test/content/folder/file.txt had an extra CR character in it, causing the infoHash to differ. Running git config core.autocrlf input and refreshing file.txt allowed the tests to pass. See here for more info: https://help.github.com/articles/dealing-with-line-endings/#platform-linux