[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/multinode-ci-nightly.yml

@@ -41,6 +41,9 @@ jobs:
   workspace:
     clean: all
   steps:
+  - task: DeleteFiles@1
+    inputs:
+      Contents: '**/*'
   - template: ${{ variables.CI_TEMPLATE_PATH }}/steps/checkout.yml@pipelines_repo
     parameters:
       submoduleBehaviour: recursive

.azuredevops/multinode-ci-pr.yml

@@ -46,6 +46,9 @@ stages:
     workspace:
       clean: all
     steps:
+    - task: DeleteFiles@1
+      inputs:
+        Contents: '**/*'
     - template: ${{ variables.CI_TEMPLATE_PATH }}/steps/checkout.yml@pipelines_repo
       parameters:
         submoduleBehaviour: recursive

.azuredevops/tests/pytest/HelloWorld.py

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