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/+/171100Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/cover/cover_test.go

@@ -81,7 +81,7 @@ var debug = flag.Bool("debug", false, "keep rewritten files for debugging")
 // We use TestMain to set up a temporary directory and remove it when
 // the tests are done.
 func TestMain(m *testing.M) {
-	dir, err := ioutil.TempDir("", "gotestcover")
+	dir, err := ioutil.TempDir("", "go-testcover")
 	if err != nil {
 		fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)