Add flow library target and wire as dep of test_operators (pytorch#18383)

Summary:
The test_operators library target globs all operator test files, but
those
files import TestFlow from executorch.backends.test.suite.flow which had
no Buck target owner. This caused listing failures for any downstream
consumer of test_operators.

Add a flow library target in test/suite/BUCK and add it as a dep of
test_operators (migrated from TARGETS to BUCK with fbcode_target).

This diff was authored with Claude.

Differential Revision: D97333154

Author: rascani

backends/test/suite/BUCK

@@ -1,4 +1,16 @@
-load("@fbcode_macros//build_defs:build_file_migration.bzl", "fbcode_target", "non_fbcode_target")
+load("@fbcode_macros//build_defs:build_file_migration.bzl", "fbcode_target")
+load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
 load(":targets.bzl", "define_common_targets")
 
-fbcode_target(_kind = define_common_targets,is_fbcode = True)
+oncall("executorch")
+
+fbcode_target(
+    _kind = runtime.python_library,
+    name = "flow",
+    srcs = ["flow.py"],
+    deps = [
+        "//executorch/backends/test/harness:tester",
+    ],
+)
+
+fbcode_target(_kind = define_common_targets, is_fbcode = True)

backends/test/suite/operators/BUCK

@@ -0,0 +1,21 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+load("@fbcode_macros//build_defs:build_file_migration.bzl", "fbcode_target")
+load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
+
+oncall("executorch")
+
+fbcode_target(
+    _kind = runtime.python_library,
+    name = "test_operators",
+    srcs = glob(["*.py"]),
+    deps = [
+        "//caffe2:torch",
+        "//executorch/backends/test/suite:flow",
+        "fbsource//third-party/pypi/pytest:pytest",
+    ],
+)