Added tests for std::fs

Details

This adds tests for the module std::fs. Tests for platform specific operations, such as obtaining the creation time of a file, are only executed when these operations are supported. The helper methods for this are currently defined in std::fs directly, but we may extract these to a separate module if necessary in the future.

Corresponding issue

#115 (closed)

Checklist

• Added tests
• Inko source code follows the Inko style guide
• Instead of using std::mirror to obtain the file path of a file, obtain the file path from the module itself.
• std::vm.stacktrace and related code should be moved to std::debug
• Extend Module so it stores the name and file path: 7e462a2a