[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.

R/pkg/inst/tests/test_sparkSQL.R

@@ -647,11 +647,11 @@ test_that("sample on a DataFrame", {
   sampled <- sample(df, FALSE, 1.0)
   expect_equal(nrow(collect(sampled)), count(df))
   expect_is(sampled, "DataFrame")
-  sampled2 <- sample(df, FALSE, 0.1)
+  sampled2 <- sample(df, FALSE, 0.1, 0) # set seed for predictable result
   expect_true(count(sampled2) < 3)
 
   # Also test sample_frac
-  sampled3 <- sample_frac(df, FALSE, 0.1)
+  sampled3 <- sample_frac(df, FALSE, 0.1, 0) # set seed for predictable result
   expect_true(count(sampled3) < 3)
 })