Added test coverage to automatic collections

refs https://github.com/TryGhost/Team/issues/3170

- The test confirms relational filters like `tag:kitchen-sink` filtering works for automatic collections

ghost/core/test/e2e-api/admin/__snapshots__/collections.test.js.snap

@@ -104,6 +104,48 @@ Object {
 }
 `;
 
+exports[`Collections API Automatic Collection Filtering Creates an automatic Collection with a relational tag filter 1: [body] 1`] = `
+Object {
+  "collections": Array [
+    Object {
+      "created_at": StringMatching /\\\\d\\{4\\}-\\\\d\\{2\\}-\\\\d\\{2\\}T\\\\d\\{2\\}:\\\\d\\{2\\}:\\\\d\\{2\\}\\\\\\.000Z/,
+      "description": "Test Collection Description with relational tag filter",
+      "feature_image": null,
+      "filter": "tag:kitchen-sink",
+      "id": StringMatching /\\[a-f0-9\\]\\{24\\}/,
+      "posts": Array [
+        Object {
+          "id": StringMatching /\\[a-f0-9\\]\\{24\\}/,
+          "sort_order": 0,
+        },
+        Object {
+          "id": StringMatching /\\[a-f0-9\\]\\{24\\}/,
+          "sort_order": 1,
+        },
+      ],
+      "title": "Test Collection with relational tag filter",
+      "type": "automatic",
+      "updated_at": StringMatching /\\\\d\\{4\\}-\\\\d\\{2\\}-\\\\d\\{2\\}T\\\\d\\{2\\}:\\\\d\\{2\\}:\\\\d\\{2\\}\\\\\\.000Z/,
+    },
+  ],
+}
+`;
+
+exports[`Collections API Automatic Collection Filtering Creates an automatic Collection with a relational tag filter 2: [headers] 1`] = `
+Object {
+  "access-control-allow-origin": "http://127.0.0.1:2369",
+  "cache-control": "no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0",
+  "content-length": "431",
+  "content-type": "application/json; charset=utf-8",
+  "content-version": StringMatching /v\\\\d\\+\\\\\\.\\\\d\\+/,
+  "etag": StringMatching /\\(\\?:W\\\\/\\)\\?"\\(\\?:\\[ !#-\\\\x7E\\\\x80-\\\\xFF\\]\\*\\|\\\\r\\\\n\\[\\\\t \\]\\|\\\\\\\\\\.\\)\\*"/,
+  "location": StringMatching /https\\?:\\\\/\\\\/\\.\\*\\?\\\\/collections\\\\/\\[a-f0-9\\]\\{24\\}\\\\//,
+  "vary": "Accept-Version, Origin, Accept-Encoding",
+  "x-cache-invalidate": "/*",
+  "x-powered-by": "Express",
+}
+`;
+
 exports[`Collections API Can add a Collection 1: [body] 1`] = `
 Object {
   "collections": Array [

ghost/core/test/e2e-api/admin/collections.test.js

@@ -379,5 +379,40 @@ describe('Collections API', function () {
                     collections: [buildMatcher(7)]
                 });
         });
+
+        it('Creates an automatic Collection with a relational tag filter', async function () {
+            const collection = {
+                title: 'Test Collection with relational tag filter',
+                description: 'Test Collection Description with relational tag filter',
+                type: 'automatic',
+                filter: 'tag:kitchen-sink'
+            };
+
+            const automaticTagCollection = await agent
+                .post('/collections/')
+                .body({
+                    collections: [collection]
+                })
+                .expectStatus(201)
+                .matchHeaderSnapshot({
+                    'content-version': anyContentVersion,
+                    etag: anyEtag,
+                    location: anyLocationFor('collections')
+                })
+                .matchBodySnapshot({
+                    collections: [buildMatcher(2)]
+                });
+
+            const kitchenSinkTagPosts = await agent
+                .get('/posts/?filter=tag:kitchen-sink');
+
+            assert.equal(automaticTagCollection.body.collections[0].posts.length, 2);
+            assert.equal(kitchenSinkTagPosts.body.posts.length, 2);
+
+            const collectionPostIds = automaticTagCollection.body.collections[0].posts.map(p => p.id);
+            const tagFilteredPosts = kitchenSinkTagPosts.body.posts.map(p => p.id);
+
+            assert.deepEqual(collectionPostIds, tagFilteredPosts);
+        });
     });
 });