* `-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
```

rb_uninterruptible() disables any interrupts using handle_interrupt
feature (This function is used by `p`).
After this function, pending interrupts should be checked correctly,
however there is no chance to setup interrupt flag of working
threads, it means that nobody checks pending interrupts.
For example, it ignores terminate signal delivered at the end
of main thread and program can't stop.

This patch set interrupt flag if there are pending interrupts.

On Deiban 9 environment, the thread tests failed and
this maximum threads information can finish up the machine
resources. To check it, I turned-off showing this information.

This has been unstable on AppVeyor mswin since the introduction
3fd83cb6fcc483d2eac0795bc139c521a3a59bd2.
https://ci.appveyor.com/project/ruby/ruby/builds/27103307/job/j7xwjmsos2k22cck

Let's have it in pending.rb to be fixed.