test/ruby/test_thread.rb: reduce thread count to avoid SIGKILL

Perhaps this error report is down to resource limits on a VM:

http://rubyci.s3.amazonaws.com/centos7/ruby-trunk/log/20181221T230003Z.fail.html.gz

But rb_mutex_t.fork_gen is still redundant, I think.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66488 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_thread.rb

@@ -1247,6 +1247,7 @@ q.pop
     run = true
     errs = ''
     nr = 50
+    nr /= 2 if Process.getrlimit(:NPROC)[0] <= 4096 # Bug 15430
     tmps = nr.times.map { Tempfile.new('Bug.15430.diagnosis') }
     thrs = nr.times.map do |_i|
       Thread.new(_i) do |i|