enhance: fix unstable unit test (#46626)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
- Core invariant: TestWait must deterministically verify that
FixedSizeAllocator.Wait() is notified when virtual resources are
released, so an allocation blocked due to exhausted virtual capacity
eventually succeeds after explicit deallocations.
- Removed/simplified logic: replaced the previous flaky pattern that
spawned 100 concurrent goroutines performing Reallocate with an explicit
channel-synchronized release goroutine that performs 100 sequential
negative Reallocate calls only after the test blocks on allocation. This
eliminates timing-dependent concurrency and the nondeterministic i-based
assertion.
- Why no data loss or behavior regression: only the test changed —
allocator implementation (Allocate/Reallocate/Release/Wait/notify) and
public APIs are unchanged. The test now exercises the same code paths
(Allocate fails, Wait blocks on cond, Reallocate/Release call notify),
but in a deterministic order, so the allocator semantics and resource
accounting (used, allocs map) remain unaffected.
- Change type and intent: Enhancement/refactor of unit test stability —
it tightens test synchronization to remove race-dependent assertions and
ensure the Wait/notify mechanism is reliably exercised without modifying
allocator logic.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: aoiasd <zhicheng.yue@zilliz.com>

pkg/util/vralloc/alloc_test.go

@@ -96,22 +96,27 @@ func TestWait(t *testing.T) {
 	a := NewFixedSizeAllocator[string](&Resource{100, 100, 100})
 	allocated, _ := a.Allocate("a1", &Resource{100, 100, 100})
 	assert.True(t, allocated)
-	for i := 0; i < 100; i++ {
-		go func(index int) {
+
+	waitCh := make(chan struct{})
+	release := func() {
+		<-waitCh
+		for i := 0; i < 100; i++ {
 			allocated, _ := a.Reallocate("a1", &Resource{-1, -1, -1})
 			assert.Equal(t, true, allocated)
-		}(i)
+		}
 	}
 
+	go release()
+
 	allocated, _ = a.Allocate("a2", &Resource{100, 100, 100})
-	i := 1
+	assert.False(t, allocated)
+	close(waitCh) // start release a1
+
 	for !allocated {
-		a.Wait()
+		a.Wait() // alloc after wait
 		allocated, _ = a.Allocate("a2", &Resource{100, 100, 100})
-		i++
 	}
 	assert.True(t, allocated)
-	assert.True(t, i < 100 && i > 1)
 }
 
 func TestPhysicalAwareFixedSizeAllocator(t *testing.T) {