Remove embedded member function for gtest in `dbms/src/Storages/Page`

Enhancement

There are some embedded member function in the directory dbms/src/Storages/Page
like PS::V3::PageStorageImpl

#ifndef NDEBUG
    // Just for tests, refactor them out later
    DB::PageStorage::SnapshotPtr getSnapshot() { return getSnapshot(""); }
    DB::PageEntry getEntry(PageId page_id) { return getEntryImpl(TEST_NAMESPACE_ID, page_id, nullptr); }
    DB::Page read(PageId page_id) { return readImpl(TEST_NAMESPACE_ID, page_id, nullptr, nullptr); }
    PageMap read(const std::vector<PageId> & page_ids) { return readImpl(TEST_NAMESPACE_ID, page_ids, nullptr, nullptr); }
    void read(const std::vector<PageId> & page_ids, const PageHandler & handler) { return readImpl(TEST_NAMESPACE_ID, page_ids, handler, nullptr, nullptr); }
    PageMap read(const std::vector<PageReadFields> & page_fields) { return readImpl(TEST_NAMESPACE_ID, page_fields, nullptr, nullptr); }
#endif

which is just for tests. This is not a elegant code style, and we need to remove them.

Start in dbms/src/Storages/Page and inspect PS::V3::PageStorageImpl, including the gtest callers of the embedded methods shown in the issue. Remove the test-only member functions and update affected tests to use the remaining interfaces; the Page storage tests should still pass without those wrappers.