playground: RIP modules, long live gosh+v23test

Also, per discussion:
- Moves 'v23test' package to x/ref/test/v23test
- Makes 'expect' package explicitly depend on 'testing', now
that 'modules' doesn't pass it an interface

MultiPart: 6/7

Change-Id: I36abb532c4b80b7a64e814793b6953468ad952b3
diff --git a/go/src/v.io/x/playground/builder/builder_v23_test.go b/go/src/v.io/x/playground/builder/builder_v23_test.go
index d792e49..2d41728 100644
--- a/go/src/v.io/x/playground/builder/builder_v23_test.go
+++ b/go/src/v.io/x/playground/builder/builder_v23_test.go
@@ -11,9 +11,9 @@
 	"testing"
 
 	"v.io/x/playground/lib/bundle/bundler"
-	"v.io/x/ref/lib/v23test"
 	_ "v.io/x/ref/runtime/factories/generic"
 	tu "v.io/x/ref/test/testutil"
+	"v.io/x/ref/test/v23test"
 )
 
 var (