Move tf.scan benchmark from contrib/eager/examples to eager/benchmarks_test.py

Eager execution is over 10x slower than defun/graph execution.

bazel run -c opt benchmarks_test -- --benchmarks=MicroBenchmarks.benchmarkScan.*

entry {
  name: "MicroBenchmarks.benchmarkScan"
  iters: 100
  wall_time: 176364.049911
  extras {
    key: "examples_per_sec"
    value {
      double_value: 5.67008979722
    }
  }
}

entry {
  name: "MicroBenchmarks.benchmarkScanDefun"
  iters: 100
  wall_time: 15466.0701752
  extras {
    key: "examples_per_sec"
    value {
      double_value: 64.6576660182
    }
  }
}

The benchmark deleted by this CL measured graph construction time, whereas this CL does not.

PiperOrigin-RevId: 212356196