Simplify is_subdir_of_any_in_list and the test that uses it
Simplify is_subdir_of_any_in_list and the test that uses it

Following the comments from sampsyo in #2494

pkess/beets,jackwilsdon/beets,sampsyo/beets,sampsyo/beets,shamangeorge/beets,jackwilsdon/beets,beetbox/beets,Kraymer/beets,shamangeorge/beets,SusannaMaria/beets,Kraymer/beets,ibmibmibm/beets,sampsyo/beets,pkess/beets,jackwilsdon/beets,shamangeorge/beets,SusannaMaria/beets,beetbox/beets,ibmibmibm/beets,jackwilsdon/beets,shamangeorge/beets,Kraymer/beets,SusannaMaria/beets,beetbox/beets,pkess/beets,beetbox/beets,ibmibmibm/beets,Kraymer/beets,sampsyo/beets,SusannaMaria/beets,ibmibmibm/beets,pkess/beets