[SPARK-20571][SPARKR][SS] Flaky Structured Streaming tests

## What changes were proposed in this pull request?

Make tests more reliable by having it till processed.
Increasing timeout value might help but ultimately the flakiness from processing delay when Jenkins is hard to account for. This isn't an actual public API supported

## How was this patch tested?
unit tests

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #17857 from felixcheung/rsstestrelia.

R/pkg/inst/tests/testthat/test_streaming.R

@@ -55,10 +55,12 @@ test_that("read.stream, write.stream, awaitTermination, stopQuery", {
   q <- write.stream(counts, "memory", queryName = "people", outputMode = "complete")
 
   expect_false(awaitTermination(q, 5 * 1000))
+  callJMethod(q@ssq, "processAllAvailable")
   expect_equal(head(sql("SELECT count(*) FROM people"))[[1]], 3)
 
   writeLines(mockLinesNa, jsonPathNa)
   awaitTermination(q, 5 * 1000)
+  callJMethod(q@ssq, "processAllAvailable")
   expect_equal(head(sql("SELECT count(*) FROM people"))[[1]], 6)
 
   stopQuery(q)
@@ -75,6 +77,7 @@ test_that("print from explain, lastProgress, status, isActive", {
   q <- write.stream(counts, "memory", queryName = "people2", outputMode = "complete")
 
   awaitTermination(q, 5 * 1000)
+  callJMethod(q@ssq, "processAllAvailable")
 
   expect_equal(capture.output(explain(q))[[1]], "== Physical Plan ==")
   expect_true(any(grepl("\"description\" : \"MemorySink\"", capture.output(lastProgress(q)))))
@@ -99,6 +102,7 @@ test_that("Stream other format", {
   q <- write.stream(counts, "memory", queryName = "people3", outputMode = "complete")
 
   expect_false(awaitTermination(q, 5 * 1000))
+  callJMethod(q@ssq, "processAllAvailable")
   expect_equal(head(sql("SELECT count(*) FROM people3"))[[1]], 3)
 
   expect_equal(queryName(q), "people3")