pgbench: Add TAP tests to check consistency of data generated

pgbench: Add TAP tests to check consistency of data generated

The tables created by pgbench rely on a few assumptions for TPC-B, where
the "filler" attribute is used to comply with this benchmark's rules as
well as pgbencn historical behavior.  The data generated for each table
uses this filler in a different way:
- pgbench_accounts uses it as a blank-padded empty string.
- pgbench_tellers and pgbench_branches use it as a NULL value.

There were no checks done about the consistency of the data initialized,
and this has showed up while discussing a patch that changes the logic
in charge of the client-side data generation (pgbench documents all that
already in its comments).  This commit adds some checks on the data
generated for both the server-side and client-side logic.

Reviewed-by: Tristan Partin
Discussion: https://postgr.es/m/ZLik4oKnqRmVCM3t@paquier.xyz