Fix broken tests when running with tf 2.0

There are tests that break when running under TF 2.0 for various reasons:

• tensorboard/plugins/debugger/debug_graphs_helper_test.py - breaks because tf.add creates nested nodes in TF 2.0 (#2583)
• tensorboard/util/encoder_test.py - Audio summary using deprecated tf.contrib.ffmpeg.encode_audio. Must find replacement. (#2557)
• tensorboard/data_compat_test.py - Audio summary using deprecated tf.contrib.ffmpeg.encode_audio. Must find replacement. (#2557)
• tensorboard/backend/event_processing/plugin_event_accumulator_test.py - Audio summary using deprecated tf.contrib.ffmpeg.encode_audio. Must find replacement. (#2558)
• tensorboard/plugins/core/core_plugin_test.py - tf.contrib.summary.* usage
• tensorboard/plugins/scalar/scalars_plugin_test.py:
  • tf.placeholder usage (#2522)
  • tf.contrib.summary.* usage for multiplexer tests (#2522)
  • tf.contrib.summary.* usage for DB writer tests (#3539)
• tensorboard/plugins/debugger/interactive_debugger_plugin_test.py (#2583)
• tensorboard/plugins/debugger/session_debug_test.py server fails to come up in TFv2. (#2583)
• tensorboard/plugins/beholder/beholder_test.py
• tensorboard/plugins/mesh/mesh_plugin_test.py (#2560)
• tensorboard/plugins/mesh/summary_test.py (#2560)
• tensorboard/data_compat_test.py (#2559)

(To generate this list, change run_v1_only to a no-op, then run all tests in TF 2.x.)

[caisq]

I can look into the debugger test failures.

[wchargin]

The last item, Beholder, was removed in #3881. We’re done!