[Azure CI] Minor environment setup fixes

- Add extra deletion to ensure source workspace is clean for the job.
- pytest expects function names to start with test_

.azuredevops/tests/pytest/HelloWorld.py

@@ -1,5 +1,5 @@
 import pytest
 
-def TestHelloWorld():
+def test_HelloWorld():
     greeting = "Hello, World!"
     assert greeting == "Hello, World!"