Skip integration tests in core CI (#677)

We already skipped history integration tests in core CI in #675 due to
suspicion on nested IRB sessions don't work on certain operating systems.

But after #669, the evaluation integration test also started to fail on
some Core CI suites. So, it looks like the integration test setup may not
work in Core CI, at least in some suites.

Consider `ruby/irb` already has rather sophisticated test suite, I think
it's better to skip the integration tests in core CI for now.

test/irb/helper.rb

@@ -87,6 +87,10 @@ def setup
         omit "Integration tests require PTY."
       end
 
+      if ruby_core?
+        omit "This test works only under ruby/irb"
+      end
+
       @envs = {}
     end
 

test/irb/test_debug_cmd.rb

@@ -10,10 +10,6 @@ class DebugCommandTest < IntegrationTestCase
     def setup
       super
 
-      if ruby_core?
-        omit "This test works only under ruby/irb"
-      end
-
       if RUBY_ENGINE == 'truffleruby'
         omit "This test runs with ruby/debug, which doesn't work with truffleruby"
       end

test/irb/test_history.rb

@@ -210,14 +210,6 @@ def with_temp_stdio
   end
 
   class NestedIRBHistoryTest < IntegrationTestCase
-    def setup
-      super
-
-      if ruby_core?
-        omit "This test works only under ruby/irb"
-      end
-    end
-
     def test_history_saving_with_nested_sessions
       write_history ""