Tests to establish errors in #1427
If there's a bad cache, fall back to downloading fresh
Persist package file extension in `WP_CLI\CoreUpgrader`

`wp core update` uses ZIP files, while `wp core download` uses gzipped
TAR files. Renaming ZIP files to `.tar.gz` causes nasty bugs. Let's keep
them cached separately.
Update cache was changed to ZIP
Merge pull request #1532 from wp-cli/fix-1427

Persist package file extension in `WP_CLI\CoreUpgrader`