Koichi Sasada 64c520fc25 skip the test now we couldn't fix
The following error is reported repeatedly on my riscv64-linux machine, so just skipt it.
I hope someone investigate it.

```
  1) Error:
TestStruct::SubStruct#test_named_structs_are_not_rooted:
Test::Unit::ProxyError: execution of Test::Unit::CoreAssertions#assert_no_memory_leak expired timeout (10 sec)
pid 1113858 killed by SIGTERM (signal 15)
| ruby 3.5.0dev (2025-05-22T21:05:12Z master 9583b7af8f) +PRISM [riscv64-linux]
|
| [7;1m1096282:1747967727.622:d70f:[mSTART={peak:453828608,size:453763072,lck:0,pin:0,hwm:9601024,rss:9601024,data:445943808,stk:135168,exe:4096,lib:7450624,pte:77824,swap:0}
| [7;1m1096282:1747967727.622:d70f:[mFINAL={peak:457502720,size:457498624,lck:0,pin:0,hwm:13312000,rss:13312000,data:449679360,stk:135168,exe:4096,lib:7450624,pte:86016,swap:0}
```
2025-05-23 15:03:10 +09:00
..
2025-04-25 13:46:05 -07:00
2025-05-23 15:03:10 +09:00
2025-04-26 11:56:42 +00:00