Reuse plugins and themes downloaded from WordPress.org across scenarios #375

Many scenarios install a plugin or theme from WordPress.org only to set up the test, such as the twentynineteen theme in the media regenerate Background or wordpress-importer before an import. Each of these installs queries the WordPress.org API and extracts the package again, which takes around 3 seconds, even when the package is already in the WP-CLI cache.

Across the bundled commands, about 430 installs per suite run are setup rather than the behavior under test, mostly in extension-command, media-command and language-command.

A step that installs a plugin or theme from a local copy after the first download would remove most of this time without changing what the scenarios test. Given these installed and active plugins: could use the same copy.