[ruby/irb] Fix test runner exit bug

(https://github.com/ruby/irb/pull/728)

* Remove useless test setup and teardown that sets MAIN_CONTEXT to nil

* Avoid adding command methods to main object in test

https://github.com/ruby/irb/commit/f204829a08

lib/irb/extend-command.rb

@@ -22,7 +22,7 @@ module IRB # :nodoc:
     #
     # Same as <code>IRB.CurrentContext.exit</code>.
     def irb_exit(ret = 0)
-      irb_context&.exit(ret)
+      irb_context.exit(ret)
     end
 
     # Displays current configuration.

test/irb/test_completion.rb

@@ -6,15 +6,6 @@ require_relative "helper"
 
 module TestIRB
   class CompletionTest < TestCase
-    def setup
-      # make sure require completion candidates are not cached
-      IRB::BaseCompletor.class_variable_set(:@@files_from_load_path, nil)
-    end
-
-    def teardown
-      IRB.conf[:MAIN_CONTEXT] = nil
-    end
-
     def completion_candidates(target, bind)
       IRB::RegexpCompletor.new.completion_candidates('', target, '', bind: bind)
     end

test/irb/test_irb.rb

@@ -733,9 +733,13 @@ module TestIRB
 
     private
 
+    def build_binding
+      Object.new.instance_eval { binding }
+    end
+
     def build_irb
       IRB.init_config(nil)
-      workspace = IRB::WorkSpace.new(TOPLEVEL_BINDING.dup)
+      workspace = IRB::WorkSpace.new(build_binding)
 
       IRB.conf[:VERBOSE] = false
       IRB::Irb.new(workspace, TestInputMethod.new)