[SPARK-18897][SPARKR] Fix SparkR SQL Test to drop test table

## 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.

(cherry picked from commit 1169db44)
Signed-off-by: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

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

@@ -205,6 +205,7 @@ test_that("create DataFrame from RDD", {
                c(16))
   expect_equal(collect(sql("SELECT height from people WHERE name ='Bob'"))$height,
                c(176.5))
+  sql("DROP TABLE people")
   unsetHiveContext()
 })