8233792: TestG1ParallelPhases.java fails with phase NonYoungFreeCSet not found (2)

Reviewed-by: lkorinth, kbarrett

test/jdk/jdk/jfr/event/gc/collection/TestG1ParallelPhases.java

@@ -176,6 +176,9 @@ class Provoker {
     public static void provokeMixedGC(int g1HeapRegionSize) {
         final var arraySize = 20_000;
         var liveOldObjects = new ArrayList<byte[]>();
+
+        // Make sure the heap is in a known state.
+        getWhiteBox().fullGC();
         allocateOldObjects(liveOldObjects, g1HeapRegionSize, arraySize);
         waitTillCMCFinished(10);
         getWhiteBox().g1StartConcMarkCycle();