Add two unit tests for tensorboard. (#9561)
Add two unit tests for tensorboard. (#9561)

It checks the number of scalars should be returned.
```
testScalars()
testScalarsCsv()
```
francoisluus/tensorboard-supervise,francoisluus/tensorboard-supervise,agrubb/tensorboard,agrubb/tensorboard,tensorflow/tensorboard,francoisluus/tensorboard-supervise,agrubb/tensorboard,tensorflow/tensorboard,qiuminxu/tensorboard,shakedel/tensorboard,ioeric/tensorboard,agrubb/tensorboard,ioeric/tensorboard,qiuminxu/tensorboard,shakedel/tensorboard,qiuminxu/tensorboard,ioeric/tensorboard,ioeric/tensorboard,tensorflow/tensorboard,francoisluus/tensorboard-supervise,qiuminxu/tensorboard,qiuminxu/tensorboard,tensorflow/tensorboard,agrubb/tensorboard,ioeric/tensorboard,tensorflow/tensorboard,shakedel/tensorboard,ioeric/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,shakedel/tensorboard,shakedel/tensorboard,francoisluus/tensorboard-supervise,qiuminxu/tensorboard,agrubb/tensorboard,francoisluus/tensorboard-supervise,shakedel/tensorboard