Backticks method invokes `/bin/sh` when the command contains
quotes, and `sh` clears some environment variables set in
runruby.rb to search the built shared library.

`make btest-ruby` is run with -q (quiet) option and -v should
remove -q option.

* `-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
```

don't duplicate the message, but prepare `out` var to choose
stdout/err.