Fix a test failure on Windows

test-all on ruby repository fails because the path doesn't have a
drive letter (C:) on Windows. Rake::Backtrace.collapse also uses
File.expand_path internally to make suppressed paths.

See:
http://ci.rubyinstaller.org/job/ruby-trunk-x64-test-all/2051/console

```
  1) Failure:
TestBacktraceSuppression#test_system_dir_suppressed [C:/Users/Worker/Jenkins/workspace/ruby-trunk-x64-build/test/rake/test_rake_backtrace.rb:19]:
Expected: []
  Actual: ["/usr/local/lib/ruby:12"]
```