add regression test
add unit test for #633

It's better to test for the specific bug, when possible.

This reverts commit d6287f7622ec71653ec0d5b4e52a8b4992183e0c.
use $p_value instead of $p_name for positional parameters
Merge pull request #633 from wp-cli/fix-633

Bug: `wp plugin install` installs a plugin when it shouldn't