In a test that does

        func TestFoo(t *testing.T) {
                defer cleanup()
                t.Fatal("oops")
        }

it can be important that cleanup run as the test fails.
The old code did this in Fatal:

        t.signal <- t
        runtime.Goexit()

The runtime.Goexit would run the deferred cleanup
but the send on t.signal would cause the main test loop
to move on and possibly even exit the program before
the runtime.Goexit got a chance to run.

This CL changes tRunner (the top stack frame of a test
goroutine) to send on t.signal as part of a function
deferred by the top stack frame.  This delays the send
on t.signal until after runtime.Goexit has run functions
deferred by the test itself.

For the above TestFoo, this CL guarantees that cleanup
will run before the test binary exits.

This is particularly important when cleanup is doing
externally visible work, like removing temporary files
or unmounting file systems.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5532078