Removed PingTest and fixed stupid _oasis conflict

PingTest was an early test that demonstated fast failover in action. It is
no longer needed. The real problem is that both PingTest and Test were
in the same directory but had different dependencies: PingTest required
Lwt and Test did not. But, since the _tags file adds dependencies by path,
Test would get linked to lwt even though it was not specified in the
_oasis file.

This is really terrible.

@seliopou you might enjoy the awful that is above
Revert "Removed PingTest and fixed stupid _oasis conflict"

This reverts commit 485838f5074355eb4be3ae736b00a79c26f0ee79.
moved ping-test to its own directory
Merge pull request #84 from frenetic-lang/lwt-conflict

Removed PingTest and fixed stupid _oasis conflict