Omit a broken https test on MinGW

This started to reliably fail on MinGW at an irrelevant commit:
https://github.com/ruby/ruby/actions/runs/6981002841/job/18997302124
https://github.com/ruby/ruby/actions/runs/6981946473/job/19000104223
https://github.com/ruby/ruby/actions/runs/6983823136/job/19005613809
https://github.com/ruby/ruby/actions/runs/6983912116/job/19005844596
https://github.com/ruby/ruby/actions/runs/6984215921/job/19006649495
https://github.com/ruby/ruby/actions/runs/6984383103/job/19007100446
https://github.com/ruby/ruby/actions/runs/6986489509/job/19012000642

So this failure is not detecting a new bug. Let's skip this until we fix
this test for MinGW.

Author: k0kubun

test/net/http/test_https.rb

@@ -167,6 +167,7 @@ def test_session_reuse
   def test_session_reuse_but_expire
     # FIXME: The new_session_cb is known broken for clients in OpenSSL 1.1.0h.
     omit if OpenSSL::OPENSSL_LIBRARY_VERSION.include?('OpenSSL 1.1.0h')
+    omit 'failing on MinGW' if /mingw/ =~ RUBY_PLATFORM
 
     http = Net::HTTP.new(HOST, config("port"))
     http.use_ssl = true