[AltTester Editor] Tests list minor assembly-related issues

## Steps to reproduce

1. Open the AltTester Unity SDK project from the GitHub repo in each Unity version (mentioned in the _Additional info_ section)
2. Open the AltTester Editor window and check the Tests list

## Actual result

### AR

1. ❌ in Unity 2020 and 2022 the test `DefaultTestExample.txt` located in `Assets/AltTester/Editor` is not displayed at all in the _Tests list_
3. ❌ in Unity 2021 the same test `DefaultTestExample.txt` is misplaced in the `AltTester.AltTesterUnitySDK` assembly
4. ❌ there is one assembly category that is not displayed in Unity 2020 and 2022 LTS: `AltTester.AltTesterUnitySDK`

### Additional info

Environment
- _SampleGame_ project with AltTester Unity SDK from `development` branch
- Unity v `2020.3.48f1` LTS, `2021.3.25f1` LTS, `2022.3.5f1` LTS

## Expected result

### ER

1. in Unity 2020 and 2022 the test `DefaultTestExample.txt` should be placed in its correspondent `AltTester.AltTesterUnitySDK.Editor` assembly category from the _Tests list_
2. in Unity 2021 the test `DefaultTestExample.txt` should be placed in its correspondent `AltTester.AltTesterUnitySDK.Editor` assembly
3. in Unity 2020 and 2022 the `AltTester.AltTesterUnitySDK` assembly category should be displayed regardless if it contains or not test scripts

### Additional info

## Testability