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>.
```
merge revision(s) 547887138f19959f649b1c0dbcde5659ae3878ed:
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67802 b2dd03c8-39d4-4d8f-98ff-823fe69b080e