http://ci.rvm.jp/results/trunk-mjit@silicon-docker/2380788
```
test_all #<Thread:0x000055b6c8e9fca8@/tmp/ruby/v2/src/trunk-mjit/tool/lib/test/unit/parallel.rb:42 run> terminated with exception (report_on_exception is true):
<internal:pack>:134:in `pack': no implicit conversion of false into String (TypeError)
from /tmp/ruby/v2/src/trunk-mjit/tool/lib/test/unit/parallel.rb:160:in `_report'
from /tmp/ruby/v2/src/trunk-mjit/tool/lib/test/unit/parallel.rb:45:in `block in _run_suite'
```
parallel test (`make test-all TESTS=-j8`) runs tests on specified
number of processes. However, some test checkers written in
`runner.rb` are not loaded. This fix enable these checkers on
parallel tests.
See also: https://github.com/ruby/ruby/pull/2508
