recent.js

module.exports = (collection) => {
  return collection
    .slice(0, 3);
};

would something like this work?

module.exports = function(eleventyConfig) {
	if(process.env.TEST_ENV) {
		eleventyConfig.addCollection("recent", function(collectionApi) {
			return collectionApi.getFilteredByGlob(["post1.md", "post2.md", "post3.md"]);
		});
	}
};

This will override collections.recent no matter how many posts have this tag assigned.

I was thinking something like this, but it feels pretty dangerous:

if (process.env.TEST_ENV) {
  eleventyConfig.addCollection('mocks', (collection) => {
    // This seems like a bad idea 😈 Creates a fake collection to mutate the collectionApi
    collection.items = collection.items.filter((item) => MOCK_POST_SLUGS.includes(item.fileSlug));
    // Have to return something so just return collection.getAll
    return collection.getAll();
  });
}

// ... now all other custom collections start with only the mock posts available to them.