[ci] Disable vstest job in Test stage. (#21492)

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#### Why I did it
Disable vstest job because sonictest agent pool is failing.
To unblock PRs.
##### Work item tracking
- Microsoft ADO **(number only)**:

#### How I did it

#### How to verify it

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azure-pipelines.yml

@@ -109,6 +109,7 @@ stages:
   - job:
     pool: sonictest
     displayName: "vstest"
+    condition: false # agent pool don't work. disable this job to unblock PRs.
     timeoutInMinutes: 60
     steps:
     - template: .azure-pipelines/cleanup.yml