appveyor.yml: do not undefine everything [ci skip]

https://ci.appveyor.com/project/ruby/ruby/builds/20432223/job/yhwbku8dg2s40e9e
C:/projects/ruby/test/lib/test/unit.rb:1004: warning: undefining `object_id' may cause serious problems
C:/projects/ruby/test/lib/test/unit.rb:1004: warning: undefining `__send__' may cause serious problems

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65891 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/excludes/_appveyor/msys2/TestEnumerable.rb

@@ -1,2 +1,2 @@
 # https://ci.appveyor.com/project/ruby/ruby/builds/20431217/job/97no5h1gqv50k1vf
-exclude(/.*/, 'This has been broken after native-fiber')
+exclude(/^test_/, 'This has been broken after native-fiber')