Add some test utils in a new entrypoint (#4127)

* Clean up README a little

This just removes some of the most egregious lies and outdated stuff. There's a
*lot* more that can be done here.

* Add some test utils in a new entrypoint

* Fix comment

* Update src/testing.ts

spec/unit/testing.spec.ts

@@ -0,0 +1,88 @@
+/*
+Copyright 2024 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { mkDecryptionFailureMatrixEvent, mkEncryptedMatrixEvent, mkMatrixEvent } from "../../src/testing";
+import { EventType } from "../../src";
+
+describe("testing", () => {
+    describe("mkMatrixEvent", () => {
+        it("makes an event", () => {
+            const event = mkMatrixEvent({
+                content: { body: "blah" },
+                sender: "@alice:test",
+                type: EventType.RoomMessage,
+                roomId: "!test:room",
+            });
+
+            expect(event.getContent()).toEqual({ body: "blah" });
+            expect(event.sender?.userId).toEqual("@alice:test");
+            expect(event.isState()).toBe(false);
+        });
+
+        it("makes a state event", () => {
+            const event = mkMatrixEvent({
+                content: { body: "blah" },
+                sender: "@alice:test",
+                type: EventType.RoomTopic,
+                roomId: "!test:room",
+                stateKey: "",
+            });
+
+            expect(event.getContent()).toEqual({ body: "blah" });
+            expect(event.sender?.userId).toEqual("@alice:test");
+            expect(event.isState()).toBe(true);
+            expect(event.getStateKey()).toEqual("");
+        });
+    });
+
+    describe("mkEncryptedMatrixEvent", () => {
+        it("makes an event", async () => {
+            const event = await mkEncryptedMatrixEvent({
+                plainContent: { body: "blah" },
+                sender: "@alice:test",
+                plainType: EventType.RoomMessage,
+                roomId: "!test:room",
+            });
+
+            expect(event.sender?.userId).toEqual("@alice:test");
+            expect(event.isEncrypted()).toBe(true);
+            expect(event.isDecryptionFailure()).toBe(false);
+            expect(event.getContent()).toEqual({ body: "blah" });
+            expect(event.getType()).toEqual("m.room.message");
+        });
+    });
+
+    describe("mkDecryptionFailureMatrixEvent", () => {
+        it("makes an event", async () => {
+            const event = await mkDecryptionFailureMatrixEvent({
+                sender: "@alice:test",
+                roomId: "!test:room",
+                code: "UNKNOWN",
+                msg: "blah",
+            });
+
+            expect(event.sender?.userId).toEqual("@alice:test");
+            expect(event.isEncrypted()).toBe(true);
+            expect(event.isDecryptionFailure()).toBe(true);
+            expect(event.getContent()).toEqual({
+                body: "** Unable to decrypt: DecryptionError: blah **",
+                msgtype: "m.bad.encrypted",
+            });
+            expect(event.getType()).toEqual("m.room.message");
+            expect(event.isState()).toBe(false);
+        });
+    });
+});