Convert boundary_conditions test to use std::async instead of fork()
Convert boundary_conditions test to use std::async instead of fork()

Allows Windows usage of this test; also cleans up code a bit. (Further
de-duping is possible but unsure if worthwhile.)


Former-commit-id: bf904bdca87d7264133d6faeb29f0e7c99439891
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide