CI: Run tests with JIT enabled, assert JIT is enabled/disabled correctly

(cherry picked from python3.15 commit 687c6ece6f633bbd9e65a680bba3e3ea80eb122c)

plan.fmf

@@ -33,6 +33,16 @@ discover:
     - name: freethreadingtest
       path: /selftest
       test: "VERSION=${pybasever}t X='-i test_check_probes -i test_sysconfigdata_json -i test_base_interpreter' ./parallel.sh"
+    - name: selftest_jit
+      path: /selftest
+      # test_attr_promotion_failure: https://github.com/python/cpython/issues/141833
+      test: "VERSION=${pybasever} PYTHON_JIT=1 X='-i test_check_probes -i test_sysconfigdata_json -i test_attr_promotion_failure' ./parallel.sh"
+    - name: jit_disabled_by_default
+      test: "python${pybasever} -c 'import sys; assert not sys._jit.is_enabled()'"
+    - name: jit_disabled_explicitly
+      test: "PYTHON_JIT=0 python${pybasever} -c 'import sys; assert not sys._jit.is_enabled()'"
+    - name: jit_enabled_explicitly
+      test: "PYTHON_JIT=1 python${pybasever} -c 'import sys; assert sys._jit.is_enabled()'"
     - name: optimizedflags
       path: /flags
       test: "python${pybasever} ./assertflags.py -O3 CFLAGS PY_BUILTIN_MODULE_CFLAGS PY_CFLAGS PY_CORE_CFLAGS PY_CFLAGS_NODIST PY_STDMODULE_CFLAGS"