debugfsinfo: improve case-sensitive testing
Previously the case-sensitive test was for the current directory, and is
fragile with errors, and could remove a real file called ".debugfsinfo".
This patch improves the case-sensitive testing so it test the given path
using a unique temporary file, and does not crash on errors.
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh