Exclude build+test files from ZIP package

[Krinkle]

Up until wp-memcached 3.0.2 there weren't any such files in the Git repository, but now that there are, we should exclude them from the package to avoid clutter in downstream embedded copies to ease auditing and reduce overall noise and size.

See also:

• https://github.com/Automattic/Co-Authors-Plus/blob/HEAD/.gitattributes
• https://github.com/Automattic/jetpack-autoloader/blob/HEAD/.gitattributes