Rename testutils to testutil and move below prometheus dir

`testutil` is more in line with stdlib naming conventions.

The package should be below `prometheus` as it only provides utils to
test exposition code, not to test HTTP client code.

Signed-off-by: beorn7 <beorn@soundcloud.com>

prometheus/testutil/testutil.go

@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-package testutils
+package testutil
 
 import (
 	"bytes"

prometheus/testutil/testutil_test.go

@@ -1,4 +1,4 @@
-package testutils
+package testutil
 
 import (
 	"testing"