Since timers are now on a P, rather than having a G running timerproc,
timejump changes to return a P rather than a G.

Updates #27707

Change-Id: I3d05af2d664409a0fd906e709fdecbbcbe00b9a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/171880
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>