The file length of env.go is getting hard to manage, so factor out the
test Runner to a new file.
Also move Runner.Close to the bottom of the file to have a more logical
progression.
Change-Id: Ifebea3277d826e9456aa02507a8349746386eb03
Reviewed-on: https://go-review.googlesource.com/c/tools/+/229780
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
