Added basic e2e test for Members API .well-known endpoint

refs https://github.com/TryGhost/Team/issues/1640
refs https://github.com/TryGhost/Members/pull/401

- Adds basic test coverage for the `GET /members/.well-known/jwks.json` endpoint
- Next the test should be expanded with the JWT verification to check if the returned format is usable by mainstream client libraries

test/e2e-api/members/__snapshots__/well-known.test.js.snap

@@ -0,0 +1,27 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`Members .well-known GET /jwks.json should return a JWKS 1: [body] 1`] = `
+Object {
+  "keys": Array [
+    Object {
+      "e": "AQAB",
+      "kid": Any<String>,
+      "kty": "RSA",
+      "n": Any<String>,
+    },
+  ],
+}
+`;
+
+exports[`Members .well-known GET /jwks.json should return a JWKS 2: [headers] 1`] = `
+Object {
+  "access-control-allow-credentials": "true",
+  "access-control-allow-origin": "http://127.0.0.1:2369",
+  "cache-control": "public, max-age=86400",
+  "content-length": "265",
+  "content-type": "application/json; charset=utf-8",
+  "etag": StringMatching /\\(\\?:W\\\\/\\)\\?"\\(\\?:\\[ !#-\\\\x7E\\\\x80-\\\\xFF\\]\\*\\|\\\\r\\\\n\\[\\\\t \\]\\|\\\\\\\\\\.\\)\\*"/,
+  "vary": "Origin, Accept-Encoding",
+  "x-powered-by": "Express",
+}
+`;

test/e2e-api/members/well-known.test.js

@@ -0,0 +1,28 @@
+const {agentProvider, fixtureManager, matchers} = require('../../utils/e2e-framework');
+const {anyString, anyEtag} = matchers;
+
+describe('Members .well-known', function () {
+    let membersAgent;
+
+    before(async function () {
+        const agents = await agentProvider.getAgentsForMembers();
+        membersAgent = agents.membersAgent;
+    });
+
+    describe('GET /jwks.json', function () {
+        it('should return a JWKS', async function () {
+            await membersAgent
+                .get('/.well-known/jwks.json')
+                .expectStatus(200)
+                .matchBodySnapshot({
+                    keys: [{
+                        kid: anyString,
+                        n: anyString
+                    }]
+                })
+                .matchHeaderSnapshot({
+                    etag: anyEtag
+                });
+        });
+    });
+});