shutdown: deflake test
shutdown: deflake test

On Linux we would occasionally see a failure like this:

--- FAIL: TestShutdown (1.06s)
    --- FAIL: TestShutdown/messy (0.03s)
        shutdown_test.go:119: child output ended, expected more lines

This was because the child process would be sent a SIGTERM before
it had a chance to register its shutdown handlers.
Fix this race condition by making the child print its "hello" message
after registering those handlers, not before.

Also remove the part of the test where we send SIGTERM to the process.
It was already shutting itself down (by calling Now itself), so there
was no reason for us to signal it. (This was its own race condition,
that was only revealed once the previous race was fixed.)

Change-Id: I7907194080ead68deeae7f1788e8a068440b4229
Reviewed-on: https://upspin-review.googlesource.com/9362
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin