Lower number of threads in a ractor in a bootstrap test

OpenBSD CI now runs inside a qemu virtual machine inside a Ubuntu
virtual machine, without hardware virtualization support. It is
limited to 2.5GB of RAM and only 2 vCPUs. There is a bootstrap test
that fails on it due to resource issues.

```
bootstraptest.test_ractor.rb_1536_1361.rb:8:in 'Thread#initialize':
can't create Thread: Cannot allocate memory (ThreadError)
```

Fix this by using 100 threads instead of 1000.

Author: jeremyevans

bootstraptest/test_ractor.rb

@@ -1534,10 +1534,10 @@ class C
 }
 
 assert_equal '1', %q{
-  N = 1_000
+  N = 100
   Ractor.new{
     a = []
-    1_000.times.map{|i|
+    100.times.map{|i|
       Thread.new(i){|i|
         Thread.pass if i < N
         a << Ractor.store_if_absent(:i){ i }