Re: pgsql: Don't trust unvalidated xl_tot_len.

Re: Thomas Munro
> In the 13 branch we see that's in the new scan_server_header()
> subroutine where it tries to open the header, after asking pg_config
> --includedir-server where it lives.  Hmm...

It's no ok to use pg_config at test time since `make install` might
not have been called yet:

https://www.postgresql.org/message-id/2023925.1644591595@sss.pgh.pa.us
https://www.postgresql.org/message-id/YqkV/hoi2SX91it8@paquier.xyz

Christoph