tests: add fakedroid as flake app

README.md

@@ -206,6 +206,45 @@ Use `nix-on-droid switch --flake .#device` to build and activate your configurat
 Therefore, every evaluation of a flake configuration will be executed with `--impure` flag. (This behaviour will be
 dropped as soon as the default setup does not require it anymore.)
 
+## Emulate `nix-on-droid`-like environment with `fakedroid`
+
+For easier debugging and testing, there is a so-called `fakedroid` environment which
+emulates the on-device environment including proot as good as possible.
+
+You can enter this emulated environment with
+
+```sh
+nix run --impure ".#fakedroid"
+```
+
+This will install nix-on-droid with initial channel setup and save the state of
+this session in the `.fakedroid` directory in the project directory. To enter this
+environment with initial flake setup, set `USE_FLAKE=1` as environment variable.
+Channel and flake setups can co-exist in the `.fakedroid` state directory. To
+rebuild/re-test the bootstrap process, remove the `.fakedroid` directory.
+
+To only run a command in fakedroid and not enter an interactive shell, just call it
+with the command as arguments:
+
+```sh
+nix run --impure ".#fakedroid" -- ls -l
+```
+
+## Testing
+
+In the [./tests/on-device](./tests/on-device) directory, there is small set
+of [bats](https://github.com/bats-core/bats-core) tests that can be executed on the
+android device or run in the `fakedroid` environment.
+
+To run the tests (on device or in fakedroid), run
+
+```sh
+nix-on-droid on-device-test
+```
+
+**Note:** This currently requires a channel setup and should only be executed on
+clean installations.
+
 ## Tips
 
 * To grant the app access to the storage, use the toggle in the app settings