## What changes were proposed in this pull request?

If spark is run with "spark.authenticate=true", then it will fail to start in local mode.

This PR generates secret in local mode when authentication on.

## How was this patch tested?

Modified existing unit test.
Manually started spark-shell.

Author: Gabor Somogyi <gabor.g.somogyi@gmail.com>

Closes #20652 from gaborgsomogyi/SPARK-23476.