Skip a flaky objspace test on Visual Studio

This seems to happen only on VisualStudio:
https://github.com/ruby/ruby/actions/runs/7130917319/job/19418375386

It fails relatively frequently. Nobody seems actively working on it, so
let's skip it until somebody starts working on it.

test/objspace/test_objspace.rb

@@ -141,6 +141,7 @@ class TestObjSpace < Test::Unit::TestCase
   end
 
   def test_reachable_objects_during_iteration
+    omit 'flaky on Visual Studio with: [BUG] Unnormalized Fixnum value' if /mswin/ =~ RUBY_PLATFORM
     opts = %w[--disable-gem --disable=frozen-string-literal -robjspace]
     assert_separately opts, "#{<<-"begin;"}\n#{<<-'end;'}"
     begin;