testing: add SignalSuite for capturing signals during tests

[davecheney]

This PR adds a SignalSuite which can be used to hook signals, like SIGQUIT,
which is generated by the timeout from the test runner.

The list of signals handled is defined in the testing.Signals variable. This is for documentation purposes, it should not be modified.

The function supplied to SignalSuite.SetUpSuite should not return. If it does, the test suite will be marked as failed unconditionally.

Usage:

   s.SignalSuite.SetUpSuite(c, func(c *gc.C, sig os.Signal) {
           fmt.Fprintf(os.Stderr, "exiting with signal: %v\n", sig)
           os.Exit(1)
   })

[jameinel]

Should this be considered outdated and closed? It didn't get a review board link.