Fix Rinda test teardown for omitted tests

New test failures on MINGW appeared after
c2e37c8.

      1) Error:
    Rinda::TupleSpaceProxyTest#test_00_template:
    NoMethodError: undefined method `stop_service' for nil:NilClass
        D:/a/ruby/ruby/src/test/rinda/test_rinda.rb:516:in `teardown'

      2) Error:
    Rinda::TupleSpaceProxyTest#test_ruby_talk_264062:
    NoMethodError: undefined method `stop_service' for nil:NilClass
        D:/a/ruby/ruby/src/test/rinda/test_rinda.rb:516:in `teardown'

Teardown happens even when the test is omitted.

See: https://github.com/ruby/ruby/runs/7058984522

test/rinda/test_rinda.rb

@@ -497,6 +497,7 @@ class TupleSpaceProxyTest < Test::Unit::TestCase
 
   def setup
     if RUBY_PLATFORM.match?(/mingw/) && ENV['MSYSTEM'] == 'UCRT64'
+      @omitted = true
       omit 'This test seems to randomly hang on GitHub Actions MinGW UCRT64'
     end
     super
@@ -506,6 +507,9 @@ def setup
     @server = DRb.start_service("druby://localhost:0")
   end
   def teardown
+    return if @omitted
+    @omitted = false
+
     # implementation-dependent
     @ts_base.instance_eval{
       if th = @keeper