Parsed test spec with 2 sessions

starting permutation: s1-begin s1-upd-ins s2-begin s2-update s1-commit s2-commit s1-result s2-result
step s1-begin: BEGIN;
step s1-upd-ins: MERGE INTO prept USING preps ON prept.t1 = preps.s1
   WHEN MATCHED THEN UPDATE SET t2 = t2 + 1
   WHEN NOT MATCHED THEN INSERT VALUES(s1, s2);
step s2-begin: BEGIN;
step s2-update: UPDATE preps SET s2 = s2 + 1; <waiting ...>
step s1-commit: COMMIT;
step s2-update: <... completed>
step s2-commit: COMMIT;
step s1-result: SELECT * FROM preps ORDER BY 1;
 s1|s2
---------------------------------------------------------------------
100| 1
200| 1
(2 rows)

step s2-result: SELECT * FROM prept ORDER BY 1;
 t1|t2
---------------------------------------------------------------------
100| 1
200| 0
(2 rows)

