Added an exception in the plugin update test for wpcomsh.

[zinigor]

Required for #37737 .

Proposed changes:

• Add an exception for the wpcomsh plugin into plugin upgrade tests.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Go to '..'

[github-actions]

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