fix: [lab1] fix incorrect pin_count in buffer_pool_test.cpp

[stackByStack]

The issue arises from two instances where a pin operation occurs: one during allocate_page and another within init_empty_page. However, there seems to be only a single unpin operation via record_page_handle.cleanup(). This discrepancy leads to a situation where cannot flush the page because the pin_count_ remains at 1, suggesting that the page is still in use. This could only be a source of frustration for those who checks the pin_count_ during page eviction.

该问题是由发生 pin 操作的两个实例引起的：一个是在 allocate_page 期间，另一个是在 init_empty_page 期间。 然而，似乎只有一个通过 record_page_handle.cleanup() 的unpin操作。 这会导致无法刷新页面的情况，因为 pin_count_ 仍为 1，表明该页面仍在使用中。 这可能只会让那些在页面驱逐期间检查 pin_count_ 的人感到困扰。

[stackByStack]

目前认为，进入evict_all_pages前page的pin_count_应为0
gdb server验证了该观点

此处的1为frame查找函数pin的，具体可查看源码，而这意味着进入evict_all_pages前page的pin_count_应为0，而test1没有达到这个预期，故应被修正