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>
