[test] Re-enable work stealing thread pool experiment (#33213)

I have not been able to reproduce the non-empty pool @ shutdown bug in
around 200k runs of various kinds. Now that experiments are marked flaky
by default, any similar failures should not block PR submission, and
this will give me good signal if the bugs reproduce more frequently in
the CI environment.

I have a fix in theory, but I don't think it should be necessary. If the
bug reproduces, I'll try the fix.

bazel/experiments.bzl

@@ -28,6 +28,7 @@ EXPERIMENTS = {
             "event_engine_listener",
             "promise_based_client_call",
             "promise_based_server_call",
+            "work_stealing",
         ],
         "cpp_end2end_test": [
             "promise_based_server_call",

src/core/lib/experiments/experiments.yaml

@@ -154,7 +154,7 @@
   default: false
   expiry: 2023/06/01
   owner: hork@google.com
-  test_tags: []
+  test_tags: ["core_end2end_test"]
   allow_in_fuzzing_config: false
 - name: client_privacy
   description: