Fix unit test for Multisite

[mukeshpanchal27]

Summary

In #956, we introduced a test suite for plugins, updating only the phpunit.xml.dist file. However, we missed updating the multisite config file tests/multisite.xml, causing it to show "No tests executed!" without an error. This PR adds the missing configuration for the multisite test suite. Thanks to @thelovekesh for spotting this oversight.

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Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>
Co-authored-by: thelovekesh <thelovekesh@git.wordpress.org>

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[thelovekesh]

Note: As discovered in #1013, wp-env based setup has some issues(#1013 (comment)). Since it's a docker-based setup we don't have much control over it. So in the same PR, I have updated the wp-env based setup with a more controllable and super-fast setup which simulates real scenario WordPress setup.