Refactor concurrency tests for basic Tree.Edit (#772)

Insert/Delete/Replace by 2 users (9 * 9 * 9 = 729 cases):
  - Ranges(9)
    - intersecting, element
    - intersecting, text
    - A contains B, element
    - A contains B, text
    - A contains B, A is element, B is text
    - side by side, element
    - side by side, text
    - A = B, element
    - A = B, text
  - Operations for both A and B(9)
    - Insert text front of range
    - Insert text middle of range
    - Insert text back of range
    - Replace to text node
    - Insert element front of range
    - Insert element middle of range
    - Insert element back of range
    - Replace to element node
    - Delete