test/rubygems/test_gem_commands_open_command.rb: prevent a warning

http://rubyci.s3.amazonaws.com/debian9/ruby-master/log/20210518T093002Z.log.html.gz
```
[ 9396/21186] TestGemCommandsOpenCommand#test_execute-e:1: warning: possibly useless use of a literal in void context
 = 0.06 s
```

test/rubygems/test_gem_commands_open_command.rb

@@ -21,7 +21,7 @@ class TestGemCommandsOpenCommand < Gem::TestCase
 
   def test_execute
     @cmd.options[:args] = %w[foo]
-    @cmd.options[:editor] = "#{Gem.ruby} -e0 --"
+    @cmd.options[:editor] = "#{Gem.ruby} -eexit --"
 
     gem 'foo', '1.0.0'
     spec = gem 'foo', '1.0.1'