tests: settings: file: merge file_littlefs

There is little point in keeping separate:
 * tests/subsys/settings/file
 * tests/subsys/settings/file_littlefs

directories, `file_littlefs` is just a thin layer used to setup test suites
for tests implemented in `file` directory.

Merge both directories together. Support for new FS can be added by simply
adding:
 * new `settings_setup_<FS_OF_CHOICE>.c` which implements
   `config_setup_fs()` function
 * expanding `testcase.yaml` to add new tests with
   `CONFIG_FILE_SYSTEM_<FS_OF_CHOICE>=y` extra config option

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