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: 852941492

Author: nvgrw

xla/backends/gpu/runtime/BUILD

@@ -955,6 +955,8 @@ xla_test(
     name = "gpublas_lt_matmul_thunk_test",
     srcs = ["gpublas_lt_matmul_thunk_test.cc"],
     backends = ["gpu"],
+    tags = ["pjrt_migration_candidate"],
+    use_legacy_runtime = True,
     deps = [
         ":gpublas_lt_matmul_thunk",
         ":thunk",
@@ -1634,6 +1636,8 @@ xla_test(
     name = "collective_broadcast_thunk_test",
     srcs = ["collective_broadcast_thunk_test.cc"],
     backends = ["gpu"],
+    tags = ["pjrt_migration_candidate"],
+    use_legacy_runtime = True,
     deps = [
         ":collective_broadcast_thunk",
         ":collective_thunk",
@@ -1718,6 +1722,8 @@ xla_test(
     name = "collective_permute_thunk_test",
     srcs = ["collective_permute_thunk_test.cc"],
     backends = ["gpu"],
+    tags = ["pjrt_migration_candidate"],
+    use_legacy_runtime = True,
     deps = [
         ":collective_permute_thunk",
         ":collective_thunk",
@@ -3594,6 +3600,8 @@ xla_test(
     name = "runtime_intrinsics_test",
     srcs = ["runtime_intrinsics_test.cc"],
     backends = ["gpu"],
+    tags = ["pjrt_migration_candidate"],
+    use_legacy_runtime = True,
     deps = [
         ":runtime_intrinsics",
         "//xla:literal",