neon/test_runner/zenith_regress

To add a new SQL test

• add sql script to run to zenith_regress/sql/testname.sql
• add expected output to zenith/regress/expected/testname.out
• add testname to both parallel_schedule and serial_schedule files*

That's it. For more complex tests see PostgreSQL regression tests. These works basically the same.

*it was changed recently in PostgreSQL upstream - no more separate serial_schedule. Someday we'll catch up with these changes.