Harden `TestObjSpace#test_memsize_of_root_shared_string`

This test occasionally fail because it runs into a String instance
that had its `==` method removed.

I couldn't identify where this String comes from, but in general
when using `each_object` it's best to not assume returned objectd
are functional.

By just inverting the operands of `==` we ensure it's always
`String#==` that is called.

```
  1) Error:
TestObjSpace#test_memsize_of_root_shared_string:
NoMethodError: undefined method '==' for #<String:0x00007f9b50e8c978>
    /tmp/ruby/src/trunk-random1/test/objspace/test_objspace.rb:35:in 'block in TestObjSpace#test_memsize_of_root_shared_string'
    /tmp/ruby/src/trunk-random1/test/objspace/test_objspace.rb:35:in 'ObjectSpace.each_object'
    /tmp/ruby/src/trunk-random1/test/objspace/test_objspace.rb:35:in 'TestObjSpace#test_memsize_of_root_shared_string'
```

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