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: 852941506

Author: nvgrw

xla/service/gpu/autotuning/BUILD

@@ -246,7 +246,9 @@ xla_test(
     tags = [
         "cuda-only",
         "no_mac",
+        "pjrt_migration_candidate",
     ],
+    use_legacy_runtime = True,
     deps = [
         ":autotune_cache_key",
         ":autotuner_util",
@@ -542,6 +544,8 @@ xla_test(
     name = "redzone_buffers_test",
     srcs = ["redzone_buffers_test.cc"],
     backends = ["gpu"],
+    tags = ["pjrt_migration_candidate"],
+    use_legacy_runtime = True,
     deps = [
         ":redzone_buffers",
         "//xla:shape_util",
@@ -630,7 +634,9 @@ xla_test(
         "cuda-only",
         "noasan",
         "nomsan",
+        "pjrt_migration_candidate",
     ],
+    use_legacy_runtime = True,
     deps = [
         ":autotuner_util",
         ":conv_algorithm_picker",
@@ -674,7 +680,10 @@ xla_cc_test(
     name = "autotuner_util_test",
     srcs = ["autotuner_util_test.cc"],
     data = ["//xla/backends/gpu/target_config:all_gpu_specs"],
-    tags = ["gpu"],
+    tags = [
+        "gpu",
+        "pjrt_migration_candidate",
+    ],
     deps = [
         ":autotune_cache_key",
         ":autotuner_status_key",