Ruby

test for IO.copy_stream CPU usage (r58534)

I'm likely to make similar mistakes in the future when working
on Fiber auto-scheduling. Start adding assertions for existing
code, first.

• test/ruby/test_io.rb (test_copy_stream_no_busy_wait): added
• test/lib/test/unit/assertions.rb (assert_cpu_usage_low): added

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58924 b2dd03c8-39d4-4d8f-98ff-823fe69b080e