Wpcomsh WPCloud testing: Make the unlinked refresh step optional.

[zinigor]

Running wp commands can break because of fatal errors in the tested code, so we're making the step optional. Rsyncing over the plugin code should fix any problems, and the refresh command will be ran the next time.

Proposed changes:

• Add return code catching to the wp dereferenced freshen command.
• Add --skip-plugins --skip-themes to the WP CLI command.

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?

No.

Testing instructions:

Either run the commands manually, or wait until CI runs.

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[anomiex]

Looks reasonable to me