[SPARK-26496][SS][TEST] Avoid to use Random.nextString in StreamingInnerJoinSuite

## What changes were proposed in this pull request?

Similar with apache#21446. Looks random string is not quite safe as a directory name.

```scala
scala> val prefix = Random.nextString(10); val dir = new File("/tmp", "del_" + prefix + "-" + UUID.randomUUID.toString); dir.mkdirs()
prefix: String = 窽텘⒘駖ⵚ駢⡞Ρ닋੎
dir: java.io.File = /tmp/del_窽텘⒘駖ⵚ駢⡞Ρ닋੎-a3f99855-c429-47a0-a108-47bca6905745
res40: Boolean = false  // nope, didn't like this one
```

## How was this patch tested?

Unit test was added, and manually.

Closes apache#23405 from HyukjinKwon/SPARK-26496.

Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: HyukjinKwon

sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamingJoinSuite.scala

@@ -350,7 +350,7 @@ class StreamingInnerJoinSuite extends StreamTest with StateStoreMetricsTest with
     withTempDir { tempDir =>
       val queryId = UUID.randomUUID
       val opId = 0
-      val path = Utils.createDirectory(tempDir.getAbsolutePath, Random.nextString(10)).toString
+      val path = Utils.createDirectory(tempDir.getAbsolutePath, Random.nextFloat.toString).toString
       val stateInfo = StatefulOperatorStateInfo(path, queryId, opId, 0L, 5)
 
       implicit val sqlContext = spark.sqlContext