Use alternative theme for testing as P2 theme has been retired #200

Bug Report

• Yes, I reviewed the contribution guidelines.
• Yes, more specifically, I reviewed the guidelines on how to write clear bug reports.

Describe the current, buggy behavior

P2 theme has been removed from the directory. Ref: https://themes.trac.wordpress.org/ticket/30322

We need to find more reliable alternative theme for feature tests.

Reference of P2 theme in our tests: https://github.com/search?q=org%3Awp-cli%20%22wp%20theme%20install%20p2%22&type=code