Make sure super is called and fix teardown

test/mkmf/base.rb

@@ -106,6 +106,7 @@ class TestMkmf < Test::Unit::TestCase
     end
 
     def teardown
+      return if @omitted
       rbconfig0 = @rbconfig
       mkconfig0 = @mkconfig
       RbConfig.module_eval {

test/mkmf/test_constant.rb

@@ -4,8 +4,10 @@ require_relative 'base'
 class TestMkmfTryConstant < TestMkmf
   def setup
     if ENV.key?('APPVEYOR')
+      @omitted = true
       omit 'This test fails too often on AppVeyor'
     end
+    super
   end
 
   def test_simple