Ignore page header when comparing VM pages in test_vm_bits.py (#7499)

## Problem

See #6714, #6967

## Summary of changes

Completely ignore page header when comparing VM pages.

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

test_runner/regress/test_vm_bits.py

@@ -168,15 +168,16 @@ def test_vm_bit_clear_on_heap_lock(neon_env_builder: NeonEnvBuilder):
     # The VM page in shared buffer cache, and the same page as reconstructed
     # by the pageserver, should be equal.
     #
-    # Ignore the LSN on the page though (first 8 bytes). If the dirty
-    # VM page is flushed from the cache for some reason, it gets WAL-logged,
-    # which changes the LSN on the page.
+    # Ignore page header (24 bytes) of visibility map.
+    # If the dirty VM page is flushed from the cache for some reason,
+    # it gets WAL-logged, which changes the LSN on the page.
+    # Also in neon SMGR we can replace empty heap page with zero (uninitialized) heap page.
     cur.execute("select get_raw_page( 'vmtest_lock', 'vm', 0 )")
-    vm_page_in_cache = (cur.fetchall()[0][0])[8:100].hex()
+    vm_page_in_cache = (cur.fetchall()[0][0])[24:100].hex()
     cur.execute(
         "select get_raw_page_at_lsn( 'vmtest_lock', 'vm', 0, pg_current_wal_insert_lsn(), NULL )"
     )
-    vm_page_at_pageserver = (cur.fetchall()[0][0])[8:100].hex()
+    vm_page_at_pageserver = (cur.fetchall()[0][0])[24:100].hex()
 
     assert vm_page_at_pageserver == vm_page_in_cache