add timeout for ev tests. 

Add settimeout(secs) function.

```lua
setloop('ev')
it('some async ev test',async,function(done)
  settimeout(0.3) -- if this test is not done within 0.3 secs, the test will fail
  ...
end)

it('some async ev test',async,function(done) 
 -- if settimeout has not been called, a default timeout of 1 second will be set for each test
  ...
end)
```