MINOR: Fix ConsumerPerformanceTest to work with non-default locales (#12623)

ConsumerPerformanceTest fails when running on a machine with a Locale where decimal is represented by a comma. E.g. in locale of Germany, one point two is written as 1,2 and not 1.2 as with the default locale.

The test fails because it validates that each header has a corresponding value by assuming that comma is a separator between values & headers. This assumption fails for Germany Locale because comma could be part of a float number.

Reviewers: David Jacot <djacot@confluent.io>

core/src/test/scala/unit/kafka/tools/ConsumerPerformanceTest.scala

@@ -25,8 +25,6 @@ import org.junit.jupiter.api.Assertions.{assertEquals, assertThrows}
 import org.junit.jupiter.api.Test
 
 class ConsumerPerformanceTest {
-
-  private val outContent = new ByteArrayOutputStream()
   private val dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss:SSS")
 
   @Test
@@ -148,6 +146,8 @@ class ConsumerPerformanceTest {
   }
 
   private def testHeaderMatchContent(detailed: Boolean, expectedOutputLineCount: Int, fun: () => Unit): Unit = {
+    val outContent = new ByteArrayOutputStream
+    try {
       Console.withOut(outContent) {
         ConsumerPerformance.printHeader(detailed)
         fun()
@@ -157,7 +157,10 @@ class ConsumerPerformanceTest {
         val header = contents(0)
         val body = contents(1)
 
-      assertEquals(header.split(",").length, body.split(",").length)
+        assertEquals(header.split(",\\s").length, body.split(",\\s").length)
+      }
+    } finally {
+      outContent.close()
     }
   }
 }