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, 4 insertions, 4 deletions

diff --git a/bootstraptest/test_thread.rb b/bootstraptest/test_thread.rb
index 38a55ff229..5361828403 100644
--- a/bootstraptest/test_thread.rb
+++ b/bootstraptest/test_thread.rb
@@ -243,7 +243,7 @@ assert_equal 'true', %{
 }
 
 assert_equal 'ok', %{
-  open("zzz.rb", "w") do |f|
+  open("zzz_t1.rb", "w") do |f|
     f.puts <<-END
       begin
         Thread.new { fork { GC.start } }.join
@@ -254,7 +254,7 @@ assert_equal 'ok', %{
       end
     END
   end
-  require "./zzz.rb"
+  require "./zzz_t1.rb"
   $result
 }
 
@@ -408,7 +408,7 @@ assert_equal 'ok', %q{
 }
 
 assert_equal 'ok', %{
-  open("zzz.rb", "w") do |f|
+  open("zzz_t2.rb", "w") do |f|
     f.puts <<-'end;' # do
       begin
         m = Thread::Mutex.new
@@ -432,7 +432,7 @@ assert_equal 'ok', %{
       end
     end;
   end
-  require "./zzz.rb"
+  require "./zzz_t2.rb"
   $result
 }