tests: settings: file_littlefs: relax dependency on LittleFS

Do some internal renaming to relax dependency on using LittleFS
specifically, making this test likely to be run on any FS.

Rename device-tree partition name `littlefs_dev` ->
`settings_file_partition`, so as does not need to be LittleFS to be
mounted and used for File settings storage.

Rename `config_setup_littlefs()` to `config_setup_fs()`, which can be
implemented for any other FS of choice.

Change FS mount point from '/littlefs' to '/fs'.

Rename `settings_test_littlefs.c` -> `settings_test_fs.c`, as this one just
defines test suites and there is nothing LittleFS specific.

Build `settings_setup_littlefs.c` only when
`CONFIG_FILE_SYSTEM_LITTLEFS=y`, so that any other FS can be added at a
later point with its specific implementation of `config_setup_fs()`
function.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>