Mark the mkfs package, which depends on a testonly group, as testonly

Change-Id: I206fab47546fcb7dd44a915e35f76700c4bc4dfb
diff --git a/BUILD.gn b/BUILD.gn
index 6e49091..34485c6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -28,6 +28,8 @@
 package("package") {
   package_name = "speccpu2000"
 
+  testonly = true
+
   deps = [
     ":speccpu2000",
     ":speccpu2000(//build/toolchain:host_x64)",