testing: add -outputdir flag so "go test" controls where the files are written

testing: add -outputdir flag so "go test" controls where the files are written
Obscure misfeature now fixed: When run from "go test", profiles were always
written in the package's source directory. This change puts them in the directory
where "go test" is run.
Also fix a couple of problems causing errors in testing.after to go unreported
unless -v was set.

R=rsc, minux.ma, iant, alex.brainman
CC=golang-dev
https://golang.org/cl/10234044