Improve speed of boundary_conditions on multicore machines
Improve speed of boundary_conditions on multicore machines

Borrow the technique from simd_op_check to improve test throughput on
typical multicore desktop machines; this reduces test runtime on my
laptop from ~30s to ~6s. Also, ensure that test functions correctly
even if assert() is disabled.


Former-commit-id: bcb76480722d1f00b84676826683892d064a8720
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide