[UT] enhance free kv cache block queue popleft_n (#24220)

Signed-off-by: Andy Xie <andy.xning@gmail.com>

tests/v1/core/test_kv_cache_utils.py

@@ -280,9 +280,11 @@ def test_free_kv_cache_block_queue_popleft_n():
     # Pop 0 block
     # fake_head->b1->b3->b5->b4->b0->b2->fake_tail
     assert len(queue.popleft_n(0)) == 0
+    assert queue.num_free_blocks == 6
     # Pop 1 block
     # fake_head->b3->b5->b4->b0->b2->fake_tail
     result_blocks = queue.popleft_n(1)
+    assert queue.num_free_blocks == 5
     assert len(result_blocks) == 1
     assert result_blocks[0] is blocks[1]
     for block in result_blocks:
@@ -292,6 +294,7 @@ def test_free_kv_cache_block_queue_popleft_n():
     # fake_head->b4->b0->b2->fake_tail
     result_blocks = queue.popleft_n(2)
     assert len(result_blocks) == 2
+    assert queue.num_free_blocks == 3
     assert result_blocks[0] is blocks[3]
     assert result_blocks[1] is blocks[5]
     for block in result_blocks:
@@ -301,6 +304,7 @@ def test_free_kv_cache_block_queue_popleft_n():
     # fake_head->fake_tail
     result_blocks = queue.popleft_n(3)
     assert len(result_blocks) == 3
+    assert queue.num_free_blocks == 0
     assert result_blocks[0] is blocks[4]
     assert result_blocks[1] is blocks[0]
     assert result_blocks[2] is blocks[2]