bootstraptest/runner: run in parallel if in parallel build

bootstraptest/runner.rb

@@ -101,8 +101,7 @@ BT = Class.new(bt) do
   end
 
   def wn=(wn)
-    if wn <= 0
-      wn = nil
+    unless wn == 1
       if /(?:\A|\s)--jobserver-(?:auth|fds)=\K(\d+),(\d+)/ =~ ENV.delete("MAKEFLAGS")
         begin
           r = IO.for_fd($1.to_i(10), "rb", autoclose: false)
@@ -112,7 +111,7 @@ BT = Class.new(bt) do
         else
           r.close_on_exec = true
           w.close_on_exec = true
-          tokens = r.read_nonblock(1024, exception: false)
+          tokens = r.read_nonblock(wn > 0 ? wn : 1024, exception: false)
           r.close
           if String === tokens
             tokens.freeze
@@ -126,7 +125,7 @@ BT = Class.new(bt) do
           end
         end
       end
-      unless wn
+      if wn <= 0
         require 'etc'
         wn = [Etc.nprocessors / 2, 1].max
       end
@@ -145,7 +144,7 @@ def main
   BT.color = nil
   BT.tty = nil
   BT.quiet = false
-  BT.wn = 1
+  # BT.wn = 1
   dir = nil
   quiet = false
   tests = nil
@@ -216,6 +215,7 @@ End
   BT.progress = %w[- \\ | /]
   BT.progress_bs = "\b" * BT.progress[0].size
   BT.tty = $stderr.tty? if BT.tty.nil?
+  BT.wn ||= /-j(\d+)?/ =~ (ENV["MAKEFLAGS"] || ENV["MFLAGS"]) ? $1.to_i : 1
 
   case BT.color
   when nil