internal/conformance: change test package name to conformance_test

Since there's no conformance package aside from the test, it's cleaner
to keep the test in the conformance_test package.

Change-Id: If9ccfdfc074fa4f390956dd17d066b8613911147
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/190981
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

internal/conformance/conformance_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package conformance
+package conformance_test
 
 import (
 	"encoding/binary"