GEODE-8814: StressNewTestHelper is not specifying multiple tests correctly (#5881)

- Multiple tests should each declare a separate `--tests` option instead
  of being comma separated.
diff --git a/geode-junit/src/main/java/org/apache/geode/test/util/StressNewTestHelper.java b/geode-junit/src/main/java/org/apache/geode/test/util/StressNewTestHelper.java
index 6806f01..f54e571 100644
--- a/geode-junit/src/main/java/org/apache/geode/test/util/StressNewTestHelper.java
+++ b/geode-junit/src/main/java/org/apache/geode/test/util/StressNewTestHelper.java
@@ -93,8 +93,7 @@
       }
 
       command.append(sourceToGradleMapping.get(sourceSet));
-      command.append(" --tests ");
-      command.append(String.join(",", entry.getValue()));
+      entry.getValue().forEach(x -> command.append(" --tests ").append(x));
       command.append(" ");
       testCount += entry.getValue().size();
     }
diff --git a/geode-junit/src/test/java/org/apache/geode/test/util/WhatExtendsJUnitTest.java b/geode-junit/src/test/java/org/apache/geode/test/util/WhatExtendsJUnitTest.java
index ea59a03..7575146 100644
--- a/geode-junit/src/test/java/org/apache/geode/test/util/WhatExtendsJUnitTest.java
+++ b/geode-junit/src/test/java/org/apache/geode/test/util/WhatExtendsJUnitTest.java
@@ -49,7 +49,7 @@
   public void classAisExtendedByBandC() {
     scanner.add(getClassLocation(A.class));
     assertThat(scanner.buildGradleCommand()).isEqualTo(
-        "repeatUnitTest --tests WhatExtendsJUnitTest$B,WhatExtendsJUnitTest$C -PtestCount=2");
+        "repeatUnitTest --tests WhatExtendsJUnitTest$B --tests WhatExtendsJUnitTest$C -PtestCount=2");
   }
 
   @Test
@@ -58,7 +58,7 @@
     scanner.add(getClassLocation(A.class));
     assertThat(scanner.buildGradleCommand())
         .isEqualTo(
-            "repeatUnitTest --tests WhatExtendsJUnitTest$B,WhatExtendsJUnitTest$C -PtestCount=2");
+            "repeatUnitTest --tests WhatExtendsJUnitTest$B --tests WhatExtendsJUnitTest$C -PtestCount=2");
   }
 
   @Test
@@ -66,7 +66,7 @@
     scanner.add(getClassLocation(A.class, "foo/src/test/java/"));
     assertThat(scanner.buildGradleCommand())
         .isEqualTo(
-            "repeatUnitTest --tests WhatExtendsJUnitTest$B,WhatExtendsJUnitTest$C -PtestCount=2");
+            "repeatUnitTest --tests WhatExtendsJUnitTest$B --tests WhatExtendsJUnitTest$C -PtestCount=2");
   }
 
   @Test
@@ -75,7 +75,7 @@
     scanner.add(getClassLocation(this.getClass(), "foo/src/integrationTest/java/"));
     assertThat(scanner.buildGradleCommand())
         .isEqualTo(
-            "repeatUnitTest --tests WhatExtendsJUnitTest$B,WhatExtendsJUnitTest$C repeatIntegrationTest --tests WhatExtendsJUnitTest -PtestCount=3");
+            "repeatUnitTest --tests WhatExtendsJUnitTest$B --tests WhatExtendsJUnitTest$C repeatIntegrationTest --tests WhatExtendsJUnitTest -PtestCount=3");
   }
 
   @Test