json: move _test.v files to vlib/json/tests/ (#22731)

vlib/json/json_decode_with_generic_test.v

@@ -1,24 +0,0 @@
-import json
-
-struct Result[T] {
-	ok     bool
-	result T
-}
-
-struct User {
-	id       int
-	username string
-}
-
-fn func[T]() !T {
-	text := '{"ok": true, "result":{"id":37467243, "username": "ciao"}}'
-	a := json.decode(Result[T], text)!
-	return a.result
-}
-
-fn test_decode_with_generic_struct() {
-	ret := func[User]()!
-	println(ret)
-	assert ret.id == 37467243
-	assert ret.username == 'ciao'
-}