gctuner: make TestGlobalMemoryTuner resilient to background GC (#68169)

close #67174

Author: zanmato1984

pkg/util/gctuner/memory_limit_tuner_test.go

@@ -107,11 +107,18 @@ func TestGlobalMemoryTuner(t *testing.T) {
 	require.Eventually(t, func() bool {
 		return GlobalMemoryLimitTuner.calcMemoryLimit(fallbackPercentage) == debug.SetMemoryLimit(-1)
 	}, 5*time.Second, 100*time.Millisecond)
-	gcNum = getNowGCNum()
+	memoryLimitGCTotal := memory.MemoryLimitGCTotal.Load()
 	memory100mb := allocator.alloc(100 << 20)
-	require.Eventually(t, func() bool {
-		return gcNum == getNowGCNum()
-	}, 5*time.Second, 100*time.Millisecond) // No GC
+	// This window used to assert "no GC at all" via NumGC, but that is inherently
+	// timing-sensitive: unrelated background GCs (or test harness GCs) can happen
+	// here without violating the intent of this test.
+	//
+	// What we really need to assert is: while MemoryLimit is in fallback mode,
+	// allocating more memory should not immediately cause an extra *memory-limit*
+	// GC/adjust cycle.
+	require.Never(t, func() bool {
+		return memoryLimitGCTotal != memory.MemoryLimitGCTotal.Load()
+	}, 500*time.Millisecond, 100*time.Millisecond)
 
 	allocator.free(memory210mb)
 	allocator.free(memory100mb)