Add flow library target and wire as dep of test_operators

[rascani]

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

[pytorch-bot]

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❌ 1 Cancelled Job, 3 Unrelated Failures

As of commit c59e54f with merge base 07b7c7e ():

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-editable / macos / macos-job (gh)
  ##[error]The operation was canceled.

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-mediatek-models-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@rascani has exported this pull request. If you are a Meta employee, you can view the originating Diff in D97333154.

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