[SPARK-28261][CORE] Fix client reuse test

## What changes were proposed in this pull request?

There is the following code in [TransportClientFactory#createClient](https://github.com/apache/spark/blob/master/common/network-common/src/main/java/org/apache/spark/network/client/TransportClientFactory.java#L150)
```
    int clientIndex = rand.nextInt(numConnectionsPerPeer);
    TransportClient cachedClient = clientPool.clients[clientIndex];
```
which choose a client from its pool randomly. If we are unlucky we might not get the max number of connections out, but less than that.

To prove that I've tried out the following test:
```java
  Test
  public void testRandom() {
    Random rand = new Random();
    Set<Integer> clients = Collections.synchronizedSet(new HashSet<>());
    long iterCounter = 0;
    while (true) {
      iterCounter++;
      int maxConnections = 4;
      clients.clear();
      for (int i = 0; i < maxConnections * 10; i++) {
        int clientIndex = rand.nextInt(maxConnections);
        clients.add(clientIndex);
      }
      if (clients.size() != maxConnections) {
        System.err.println("Unexpected clients size (iterCounter=" + iterCounter + "): " + clients.size() + ", maxConnections: " + maxConnections);
      }
      if (iterCounter % 100000 == 0) {
        System.out.println("IterCounter: " + iterCounter);
      }
    }
  }
```

Result:
```
Unexpected clients size (iterCounter=22388): 3, maxConnections: 4
Unexpected clients size (iterCounter=36244): 3, maxConnections: 4
Unexpected clients size (iterCounter=85798): 3, maxConnections: 4
IterCounter: 100000
Unexpected clients size (iterCounter=97108): 3, maxConnections: 4
Unexpected clients size (iterCounter=119121): 3, maxConnections: 4
Unexpected clients size (iterCounter=129948): 3, maxConnections: 4
Unexpected clients size (iterCounter=173736): 3, maxConnections: 4
Unexpected clients size (iterCounter=178138): 3, maxConnections: 4
Unexpected clients size (iterCounter=195108): 3, maxConnections: 4
IterCounter: 200000
Unexpected clients size (iterCounter=209006): 3, maxConnections: 4
Unexpected clients size (iterCounter=217105): 3, maxConnections: 4
Unexpected clients size (iterCounter=222456): 3, maxConnections: 4
Unexpected clients size (iterCounter=226899): 3, maxConnections: 4
Unexpected clients size (iterCounter=229101): 3, maxConnections: 4
Unexpected clients size (iterCounter=253549): 3, maxConnections: 4
Unexpected clients size (iterCounter=277550): 3, maxConnections: 4
Unexpected clients size (iterCounter=289637): 3, maxConnections: 4
...
```

In this PR I've adapted the test code not to have this flakyness.

## How was this patch tested?

Additional (not committed test) + existing unit tests in a loop.

Closes apache#25075 from gaborgsomogyi/SPARK-28261.

Authored-by: Gabor Somogyi <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: gaborgsomogyi

common/network-common/src/test/java/org/apache/spark/network/TransportClientFactorySuite.java

@@ -117,7 +117,7 @@ private void testClientReuse(int maxConnections, boolean concurrent)
       }
 
       Assert.assertEquals(0, failed.get());
-      Assert.assertEquals(clients.size(), maxConnections);
+      Assert.assertTrue(clients.size() <= maxConnections);
 
       for (TransportClient client : clients) {
         client.close();