The current build system uses runtime dependencies from only
`.bundle` directory. We shouldn't install runtime dependencies
from rubygems.org when `make test-bundled-gems` is invoked.
To use the repository version of bundled gems, we need to build a gem by
"gem build", but the repository of minitest does not include
minitest.gemspec because it uses hoe.
This change creats a dummy minitest.gemspec to pass the CI.
Gemspec files generated by old bundler run `git` without changing the
working directory.
Or some gemspec files expect an owned file at the top exists ath the
current working directory.
