Update unit tests docs (#3977)

Co-authored-by: あく <alleteam@gmail.com>

documentation/UnitTests.md

@@ -20,15 +20,15 @@ To run the unit tests, follow these steps:
 3. Launch the CLI session and run the `unit_tests` command.
 
 **NOTE:** To run a particular test (and skip all others), specify its name as the command argument.
-See [test_index.c](https://github.com/flipperdevices/flipperzero-firmware/blob/dev/applications/debug/unit_tests/test_index.c) for the complete list of test names.
+Test names match application names defined [here](https://github.com/flipperdevices/flipperzero-firmware/blob/dev/applications/debug/unit_tests/application.fam).
 
 ## Adding unit tests
 
 ### General
 
 #### Entry point
 
-The common entry point for all tests is the [unit_tests](https://github.com/flipperdevices/flipperzero-firmware/tree/dev/applications/debug/unit_tests) app. Test-specific code is placed into an arbitrarily named subdirectory and is then called from the [test_index.c](https://github.com/flipperdevices/flipperzero-firmware/tree/dev/applications/debug/unit_tests/test_index.c) source file.
+The common entry point for all tests is the [unit_tests](https://github.com/flipperdevices/flipperzero-firmware/tree/dev/applications/debug/unit_tests) app. Test-specific code is packaged as a `PLUGIN` app placed in a subdirectory of `tests` in the `unit_tests` mother-app and referenced in the common `application.fam`. Look at other tests for an example.
 
 #### Test assets