Move problem 1-2 (join) into project 2 as the "wait" system call.

[pintos-anon] / grading / vm / page-parallel.exp

(page-parallel) begin
(page-parallel) start child 0
(page-parallel) start child 1
(page-parallel) start child 2
(page-parallel) wait for child 0
(page-parallel) wait for child 1
(page-parallel) wait for child 2
(page-parallel) end