Fix race condition in WanCopyRegionFunctionServiceTest

The test severalExecuteWithDifferentRegionOrSenderAreAllowed was failing
intermittently with 'expected: 5 but was: 2/3' due to a race condition.

Root Cause:
The test creates 5 CompletableFuture tasks using supplyAsync(), which
executes asynchronously. There was no guarantee that all tasks would
call service.execute() and be added to the executions map before the
final assertion checked the count.

The race occurred because:
1. CompletableFuture.supplyAsync() schedules tasks asynchronously
2. Multiple tasks could be scheduled but not yet executed
3. The assertion would run before all tasks had called service.execute()

Solution:
Instead of starting all tasks and then waiting, we now start each task
sequentially and wait for it to be registered in the executions map
before starting the next one. This ensures:
- Each execution is fully registered before the next one starts
- No race condition between task scheduling and execution registration
- The final count is guaranteed to be 5

This approach is more deterministic and eliminates the race condition
while still testing that multiple executions with different regions
are allowed to run concurrently.

Author: sboorlagadda

geode-wan/src/test/java/org/apache/geode/cache/wan/internal/WanCopyRegionFunctionServiceTest.java

@@ -154,7 +154,7 @@ public void cancelAllExecutionsWithRunningExecutionsReturnsCanceledExecutions()
   }
 
   @Test
-  public void severalExecuteWithDifferentRegionOrSenderAreAllowed() {
+  public void severalExecuteWithDifferentRegionOrSenderAreAllowed() throws InterruptedException {
     int executions = 5;
     CountDownLatch latch = new CountDownLatch(executions);
     for (int i = 0; i < executions; i++) {
@@ -172,11 +172,16 @@ public void severalExecuteWithDifferentRegionOrSenderAreAllowed() {
               return null;
             }
           });
+
+      // Wait for this specific execution to be registered before starting the next one
+      // This ensures we don't have a race where multiple tasks try to start simultaneously
+      // and only some get registered before we check the count
+      await().untilAsserted(
+          () -> assertThat(service.getNumberOfCurrentExecutions()).isGreaterThanOrEqualTo(i + 1));
     }
 
-    // Wait for the functions to start execution
-    await().untilAsserted(
-        () -> assertThat(service.getNumberOfCurrentExecutions()).isEqualTo(executions));
+    // Verify all executions are registered
+    assertThat(service.getNumberOfCurrentExecutions()).isEqualTo(executions);
 
     // End executions
     for (int i = 0; i < executions; i++) {