Stop reusing fd

[soutaro]

An error is reported in ruby CI.

Error: test_file?(FileTestSingletonTest): Errno::EBADF: Bad file descriptor
  test/stdlib/FileTest_test.rb:13:in `close'

We find there are reusing a fd in test. So, we assume the EBADF is happening by:

1. sysopen in IO_test returns a file descriptor N
2. N is passed to two IO.open calls during test
3. The first IO instance is closed by GC, but the second one is still alive
4. The sysopen in FileTest_test returns N, so another IO instance with the file descriptor is created
5. There are two IO instances associated to N, one created in IO_test, other created in FileTest_test
6. IO is closed by GC successfully, because N is opened in FileTest_test
7. Calling close in FileTest_test raises an error, because N is already closed

We hope this fix solves the problem. 🤞