Add more assertions in test_uplus_minus

Not along after 1b83074 CI
started to rarely fail this test:

```
TestString#test_uplus_minus: Test::Unit::AssertionFailedError: uminus deduplicates [Feature #13077].
  1) Failure:
TestString#test_uplus_minus [/tmp/ruby/src/trunk/test/ruby/test_string.rb:3368]:
```

It's unclear what is going on, but one possibility is that
`"bar".freeze` might no longer compile correctly.

Another possibility is that another test redefine `String#freeze`,
causing `opt_str_freeze` to no longer return an `fstring`.

test/ruby/test_string.rb

@@ -3356,16 +3356,20 @@ def test_uplus_minus
     assert_same(str, +str)
     assert_not_same(str, -str)
 
+    require 'objspace'
+
     str = "bar".freeze
+    assert_includes ObjectSpace.dump(str), '"fstring":true'
+
     assert_predicate(str, :frozen?)
     assert_not_predicate(+str, :frozen?)
     assert_predicate(-str, :frozen?)
 
     assert_not_same(str, +str)
     assert_same(str, -str)
 
-    bar = %w(b a r).join('')
-    assert_same(str, -bar, "uminus deduplicates [Feature #13077]")
+    bar = -%w(b a r).join('')
+    assert_same(str, bar, "uminus deduplicates [Feature #13077] #{ObjectSpace.dump(bar)}")
   end
 
   def test_uminus_frozen