[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 <[email protected]>

Closes #17903 from falaki/SPARK-20661.

(cherry picked from commit 2abfee1)
Signed-off-by: Yin Huai <[email protected]>

Author: falaki

R/pkg/inst/tests/testthat/test_sparkSQL.R

@@ -668,6 +668,8 @@ test_that("jsonRDD() on a RDD with json string", {
 })
 
 test_that("test tableNames and tables", {
+  # Making sure there are no registered temp tables from previous tests
+  suppressWarnings(sapply(tableNames(), function(tname) { dropTempTable(tname) }))
   df <- read.json(jsonPath)
   createOrReplaceTempView(df, "table1")
   expect_equal(length(tableNames()), 1)