Tag PjRt migration candidates explicitly. (BUILD changes)

This change adds a new tag "pjrt_migration_candidate" to all test targets that
depend on HloTestBase, ClientLibraryTestBase, and HloRunnerTpuSystem.

This change also adds a new `use_legacy_runtime` kwarg to `xla_test`, which acts
as a replacement for "test_migrated_to_hlo_runner_pjrt".  During a brief
transition phase, we will leave all "test_migrated_to_hlo_runner_pjrt" tags in
place so that we can identify any tests that have the `use_legacy_runtime` set
to an incorrect value.

"pjrt_migration_candidate" and "test_migrated_to_hlo_runner_pjrt" are mutually
exclusive.

"pjrt_migration_candidate" should not appear on any tests using the new runtime.

Unlike "test_migrated_to_hlo_runner_pjrt", which primarily tags `xla_test`
targets, "pjrt_migration_candidate" intends to tag all outstanding migration
candidates to obtain an accurate picture of migration progress. If a test cannot
or should not be migrated, it can be excluded from any analysis just by removing
the tag.

PiperOrigin-RevId: 853305489

Author: nvgrw

xla/stream_executor/sycl/BUILD

@@ -152,7 +152,9 @@ xla_test(
     tags = [
         "gpu",
         "oneapi-only",
+        "pjrt_migration_candidate",
     ],
+    use_legacy_runtime = True,
     deps = [
         ":sycl_event",
         ":sycl_platform_id",