* `-j` option for concurrent test with threads
* `-jN` uses N threads
* `-j` uses nproc/2 threads
* Introduce `BT` struct to manage configurations
* Introduce `Assertion` to manage all assertions
* Remove all toplevel instance variables
* Show elapsed seconds at last
```
$ time make btest
...
real 0m37.319s
user 0m26.221s
sys 0m16.534s
$ time make btest TESTOPTS=-j
...
real 0m11.812s
user 0m36.667s
sys 0m21.872s
```
Koichi Sasada