test/ruby/test_fiber.rb: reduce the count of object creation to cause GC

The test consistently fails on OpenBSD.

https://rubyci.org/logs/rubyci.s3.amazonaws.com/openbsd-current/ruby-master/log/20190903T010009Z.fail.html.gz
```
  1) Failure:
TestFiber#test_fork_from_fiber [/home/chkbuild/chkbuild/tmp/build/20190903T010009Z/ruby/test/ruby/test_fiber.rb:327]:
[ruby-core:41456].
<0> expected but was
<1>.
```

Author: mame

test/ruby/test_fiber.rb

@@ -311,7 +311,9 @@ def test_fork_from_fiber
           Fiber.new {
             xpid = fork do
               # enough to trigger GC on old root fiber
-              10000.times do
+              count = 10000
+              count = 1000 if /openbsd/i =~ RUBY_PLATFORM
+              count.times do
                 Fiber.new {}.transfer
                 Fiber.new { Fiber.yield }
               end