[SPARK-9865][SPARKR] Flaky SparkR test: test_sparkSQL.R: sample on a DataFrame
[SPARK-9865][SPARKR] Flaky SparkR test: test_sparkSQL.R: sample on a DataFrame

Make sample test less flaky by setting the seed

Tested with
```
repeat {  if (count(sample(df, FALSE, 0.1)) == 3) { break } }
```

Author: felixcheung <felixcheung_m@hotmail.com>

Closes #9549 from felixcheung/rsample.

(cherry picked from commit cd174882a5a211298d6e173fe989d567d08ebc0d)
Signed-off-by: Shivaram Venkataraman <ad63b47dfac1951bde2b661ba9fbbe123f686bee@cs.berkeley.edu>

haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark