[SPARK-26120][TESTS][SS][SPARKR] Fix a streaming query leak in Structured Streaming R tests

## What changes were proposed in this pull request?

Stop the streaming query in `Specify a schema by using a DDL-formatted string when reading` to avoid outputting annoying logs.

## How was this patch tested?

Jenkins

Closes apache#23089 from zsxwing/SPARK-26120.

Authored-by: Shixiong Zhu <zsxwing@gmail.com>
Signed-off-by: hyukjinkwon <gurwls223@apache.org>
(cherry picked from commit 4b7f7ef)
Signed-off-by: hyukjinkwon <gurwls223@apache.org>

Author: zsxwing

R/pkg/tests/fulltests/test_streaming.R

@@ -127,6 +127,7 @@ test_that("Specify a schema by using a DDL-formatted string when reading", {
   expect_false(awaitTermination(q, 5 * 1000))
   callJMethod(q@ssq, "processAllAvailable")
   expect_equal(head(sql("SELECT count(*) FROM people3"))[[1]], 3)
+  stopQuery(q)
 
   expect_error(read.stream(path = parquetPath, schema = "name stri"),
                "DataType stri is not supported.")