CI: ignore test 286 on Appveyor gcc 9 build

This test fails sometimes with a super fast retry loop due to what may
just be a compiler bug. The test results are ignored on the one CI job
where it occurs because there seems to be nothing we can do to fix it.

Fixes #12040
Closes #12106

appveyor.yml

@@ -157,7 +157,8 @@ environment:
       ENABLE_UNICODE: 'OFF'
       HTTP_ONLY: 'OFF'
       TESTING: 'ON'
-      DISABLED_TESTS: '!1086 !1139 !1451 !1501'
+      # test 286 disabled due to https://github.com/curl/curl/issues/12040
+      DISABLED_TESTS: '~286 !1086 !1139 !1451 !1501'
       ADD_PATH: 'C:\msys64\mingw64\bin;C:\msys64\usr\bin'
       MSYS2_ARG_CONV_EXCL: '/*'
       BUILD_OPT: -k