## What changes were proposed in this pull request?
SparkR tests, `R/run-tests.sh`, succeeds only once because `test_sparkSQL.R` does not clean up the test table, `people`.
As a result, the rows in `people` table are accumulated at every run and the test cases fail.
The following is the failure result for the second run.
```r
Failed -------------------------------------------------------------------------
1. Failure: create DataFrame from RDD (test_sparkSQL.R#204) -------------------
collect(sql("SELECT age from people WHERE name = 'Bob'"))$age not equal to c(16).
Lengths differ: 2 vs 1
2. Failure: create DataFrame from RDD (test_sparkSQL.R#206) -------------------
collect(sql("SELECT height from people WHERE name ='Bob'"))$height not equal to c(176.5).
Lengths differ: 2 vs 1
```
## How was this patch tested?
Manual. Run `run-tests.sh` twice and check if it passes without failures.
Author: Dongjoon Hyun <dongjoon@apache.org>
Closes#16310 from dongjoon-hyun/SPARK-18897.
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