Use env var to turn on YJIT for test-bundled-gems

tool/test-bundled-gems.rb use sub processes for testing bundled gems and
doesn't support RUN_OPTS. We weren't enabling YJIT for these tests.
Use an include config with RUBY_YJIT_ENABLE to turn on YJIT for these
tests.

Note that we only test with the default call threshold in this setup,
which is the same as before YJIT was off by defauft. The
--yjit-call-threshold command line was never passed to the tests.

.github/workflows/yjit-ubuntu.yml

@@ -4,7 +4,7 @@ jobs:
   make:
     strategy:
       matrix:
-        test_task: ["check", "test-bundled-gems"] # "test-bundler-parallel",
+        test_task: ["check"] # "test-bundler-parallel",
         os:
           - ubuntu-20.04
 #         - ubuntu-18.04
@@ -20,6 +20,10 @@ jobs:
             os: ubuntu-20.04
             configure: ""
             yjit_enable_env: RUBY_YJIT_ENABLE
+          - test_task: "test-bundled-gems"
+            os: ubuntu-20.04
+            configure: "cppflags=-DRUBY_DEBUG"
+            yjit_enable_env: RUBY_YJIT_ENABLE
       fail-fast: false
     env:
       GITPULLOPTIONS: --no-tags origin ${{github.ref}}