feat: randomly allocate a isolation level for txn when create test case

[gukj-spel]

pr for issue #42, randomly allocate a isolation level for every txn when create test case

• add isolation level message in description
• isolation level: READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ,SERIALIZABLE
• for example: II0-ICI1-ICI2.txt , the description is follow

  #
  # Test case description
  # POPG Pattern: T1 ==II0==> T2 ==ICI1==> T3 ==ICI2==> T1
  # Parameters: #column=2 #txn=3 #operations=3 #variable=3
  # Structure: Sequence-Session-Query
  # Txn Isolation: T1:REPEATABLE-READ,T2:READ-COMMITTED,T3:REPEATABLE-READ
  # When sequence=0, it is a preparation phase, otherwise an execution phase
  #