support concurrent btest execution

* `-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
```

1 files changed, 1 insertions, 1 deletions

diff --git a/bootstraptest/test_fiber.rb b/bootstraptest/test_fiber.rb
index f651050b96..2614dd13bf 100644
--- a/bootstraptest/test_fiber.rb
+++ b/bootstraptest/test_fiber.rb
@@ -10,7 +10,7 @@ show_limit %q{
     puts "Fiber count: #{fibers.count} (#{error})"
     break
   end while true
-} unless @quiet
+}
 
 assert_equal %q{ok}, %q{
   Fiber.new{