Try to fix NoMethodError on slow environments

```
  1) Failure:
TestParallel::TestParallel#test_hungup [/home/user/ruby/tool/test/testunit/test_parallel.rb:215]:
Expected /^Retrying hung up testcases\.+$/ to match "Run options: \n" +
"  --seed=43403\n" +
"  --ruby\n" +
"  \"./miniruby -I../lib -I. -I.ext/common ../tool/runruby.rb --extout=.ext -- --disable-gems\"\n" +
"  -j\n" +
"  t1\n" +
"  --worker-timeout=1\n" +
"\n" +
"# Running tests:\n" +
"\n" +
"/home/user/ruby/tool/lib/test/unit.rb:687:in `block in _run_parallel': undefined method `<' for nil:NilClass (NoMethodError)\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:538:in `block in quit_workers'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:537:in `reject!'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:537:in `quit_workers'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:687:in `_run_parallel'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:810:in `_run_suites'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:849:in `_run_suites'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:1479:in `_run_anything'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:1263:in `_run_anything'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:1654:in `run_tests'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:1641:in `block in _run'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:1640:in `each'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:1640:in `_run'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:1682:in `run'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:1034:in `run'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:882:in `run'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:154:in `run'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:1761:in `run'\n" +
"\tfrom /home/user/ruby/tool/lib/test/unit.rb:1765:in `run'\n" +
"\tfrom /home/user/ruby/tool/test/testunit/tests_for_parallel/runner.rb:14:in `<main>'\n".
```

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