[SPARK-20661][SPARKR][TEST] SparkR tableNames() test fails
[SPARK-20661][SPARKR][TEST] SparkR tableNames() test fails

## What changes were proposed in this pull request?
Cleaning existing temp tables before running tableNames tests

## How was this patch tested?
SparkR Unit tests

Author: Hossein <hossein@databricks.com>

Closes #17903 from falaki/SPARK-20661.

(cherry picked from commit 2abfee18b6511482b916c36f00bf3abf68a59e19)
Signed-off-by: Yin Huai <8adc4758e0403b3bd961e4e414291eaa9ba64098@databricks.com>

jlopezmalla/spark,apache-spark-on-k8s/spark,publicRoman/spark,jlopezmalla/spark,apache-spark-on-k8s/spark,publicRoman/spark,jlopezmalla/spark,publicRoman/spark,apache-spark-on-k8s/spark,apache-spark-on-k8s/spark,publicRoman/spark,apache-spark-on-k8s/spark,publicRoman/spark,jlopezmalla/spark,publicRoman/spark,apache-spark-on-k8s/spark,jlopezmalla/spark,apache-spark-on-k8s/spark,jlopezmalla/spark,jlopezmalla/spark,publicRoman/spark