Merge branch 'js/mingw-is-hidden-test-fix'

A Windows-specific test element has been made more robust against
misuse from both user's environment and programmer's errors.

* js/mingw-is-hidden-test-fix:
  t: restrict `is_hidden` to be called only on Windows
  mingw: make test_path_is_hidden more robust
  t: consolidate the `is_hidden` functions

diff --cc t/test-lib-functions.sh
Simple merge