"TracePoint.enable" is implemented in prelude.rb since
r66003 / commit 96990203
and not available in miniruby.  I tried using regular "ruby"
for testing, but it proved noisy and caused test failures.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66336 b2dd03c8-39d4-4d8f-98ff-823fe69b080e