issues with test_paths

[mathause]

test_paths - i.e. passing a list of strings (instead of querying the filesystem) can fail:

1. Non-unique paths can be passed.

   from filefisher import FileFinder
   
   ff = FileFinder(
       path_pattern="{cat}",
       file_pattern="{file}",
       test_paths=["a/file1", "a/file1"]
   )
   
   ff.find_files() # fails (correctly)

2. find_paths can fail with seemingly duplicated paths:

   from filefisher import FileFinder
   
   ff = FileFinder(
       path_pattern="{cat}",
       file_pattern="{file}",
       test_paths=["a/file1", "a/file2"]
   )
   
   ff.find_files() # works
   ff.find_paths() # fails (but should not)

not a blocker because it does not affect the "real" functionality.