Zuul cloner for extensions is now a macro

Invocation of Zuul cloner for mediawiki extensions is hardcoded in the
PHPUnit template '{name}-{ext-name}-testextension'. Move those bits to a
new builder macro 'zuul-cloner-extdeps' so we can reuse the logic
elsewhere, such as for qunit jobs.

Change is a noop.

Change-Id: I0cff9e08f4b29180ca8e856da66584931811febb