cmd/cover: rename temporary directory prefix for consistency

This change renames the temporary directory prefix for testing to
go-testcover from gotestcover. It looks like other packages have the
"go-" prefix for temporary directories, such as go-build, go-tool-dist
and go-nettest.

Change-Id: I91ab570d33c4c1bb48e6e01451a811272f6f8b77
Reviewed-on: https://go-review.googlesource.com/c/go/+/171100


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>