Use a single synapse for all unread tests. (#11489)

Saves about 30 seconds per run.

cypress/e2e/read-receipts/high-level.spec.ts

@@ -31,10 +31,26 @@ describe("Read receipts", () => {
     let alphaRoomId: string;
     let bot: MatrixClient | undefined;
 
-    beforeEach(() => {
+    before(() => {
-        // Create 2 rooms: Alpha & Beta. We join the bot to both of them
+        // Note: unusually for the Cypress tests in this repo, we share a single
+        // Synapse between all the tests in this file.
+        //
+        // Stopping and starting Synapse costs about 0.25 seconds per test, so
+        // for most suites this is worth the cost for the extra assurance that
+        // each test is independent.
+        //
+        // Because there are so many tests in this file, and because sharing a
+        // Synapse should have no effect (because we create new rooms and users
+        // for each test), we share it here, saving ~30 seconds per run at time
+        // of writing.
+
         cy.startHomeserver("default").then((data) => {
             homeserver = data;
+        });
+    });
+
+    beforeEach(() => {
+        // Create 2 rooms: Alpha & Beta. We join the bot to both of them
         cy.initTestUser(homeserver, userName)
             .then(() => {
                 cy.createRoom({ name: roomAlpha }).then((createdRoomId) => {
@@ -62,9 +78,8 @@ describe("Read receipts", () => {
                 cy.findByText(botName + " joined the room").should("exist");
             });
     });
-    });
 
-    afterEach(() => {
+    after(() => {
         cy.stopHomeserver(homeserver);
     });