how to avoid duplication when object becomes subject in second half sentence: X has an Y, and this Y does Z

One concise way to achieve that is to transform the predicate adjectival part into an attributive one, as shown now.

• A software project should comprise an ever growing test suite.

In the way of keeping to two separate specifications there is also the possibility of using the idiomatic phrase "at that" with pronoun support (one), but if it is not considered informal by OALD, it is so by the Cambridge dictionary. It follows that you can expect it to be at least slightly informal.

• A software project should comprise a test suite—and at that an ever growing one.

• A software project should comprise a test suite—and an ever growing one at that.

Pronoun support is also possible with the second form, where no ambiguity can result from context.

• A software project should comprise a test suite that should be an ever growing one.