Steps to reproduce :
1. Create a new Project
2. Open the Test Runner (Window > General > Test Runner)
3. In the Project window, create a new folder in the “Assets” folder
4. Open the created folder
5. Press “Create a new Test Assembly Folder in the active path.” in the Test Runner window
6. Open the “Tests” folder
7. Select the “Tests.asmdef” file
8. Press “Create a new Test Script in the active path.” in the Test Runner window
9. Observe the console

Expected result: A test script is created in the same directory as the selected file
Actual result: A new test script is not created and an error is thrown “Invalid generated unique path '' (input path 'Assets/Scripts/Tests/Tests.asmdef/NewTestScript.cs')”

Reproducible with: 2023.1.0a1, 6000.0.59f1, 6000.2.7f1, 6000.3.0b4, 6000.4.0a1

Reproducible on: Windows 11 Pro (24H2)
Not reproducible on: No other environments tested

Note: In com.unity.test-framework 1.4.5 the issue does not reproduce