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 Pending, 2 Unrelated Failures

As of commit e9d083c with merge base 7c79395 ():

BROKEN TRUNK - The following jobs failed but were 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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