Isolate test_gc_compact on msys2 AppVeyor

as it's unstable on the environment
https://ci.appveyor.com/project/ruby/ruby/builds/24138134/job/i7e441u7se11w7ey

appveyor.yml

@@ -109,9 +109,9 @@ for:
     - if not "%GEMS_FOR_TEST%" == "" ..\install\bin\gem install --no-document %GEMS_FOR_TEST%
   test_script:
     - mingw32-make test
-    - mingw32-make test-all TESTOPTS="--retry --job-status=normal --show-skip --subprocess-timeout-scale=1.5 --excludes=../ruby/test/excludes/_appveyor -j %JOBS% --exclude win32ole --exclude test_open-uri"
+    - mingw32-make test-all TESTOPTS="--retry --job-status=normal --show-skip --subprocess-timeout-scale=1.5 --excludes=../ruby/test/excludes/_appveyor -j %JOBS% --exclude win32ole --exclude test_open-uri --exclude test_gc_compact"
     # separately execute tests without -j which may crash worker with -j.
-    - mingw32-make test-all TESTOPTS="--retry --job-status=normal --show-skip --subprocess-timeout-scale=1.5 --excludes=../ruby/test/excludes/_appveyor" TESTS="../ruby/test/win32ole ../ruby/test/open-uri/test_open-uri.rb"
+    - mingw32-make test-all TESTOPTS="--retry --job-status=normal --show-skip --subprocess-timeout-scale=1.5 --excludes=../ruby/test/excludes/_appveyor" TESTS="../ruby/test/win32ole ../ruby/test/open-uri/test_open-uri.rb ../ruby/test/ruby/test_gc_compact.rb"
     - mingw32-make test-spec MSPECOPT=-fs # not using `-j` because sometimes `mspec -j` silently dies on Windows
 notifications:
   # Using "Webhook" with templated body to skip notification on Pull Request