Specify a 'run dir' rather than relying on pwd all over.

- Create dummy app in spec/dummy for running tests; avoids interference of MetricFu file system details with testing
- Replace all Dir.pwd with MetricFu.run_dir, which defaults to Dir.pwd but can be set
- Add MetricFu.run_path, which is Pathname(MetricFu.run_dir), and replaces Pathname.pwd where used