test: the test suite exits on its own, without --forceExit (closes #585)

Some tests left timers running when they ended, and --forceExit hid them.

- Six background test files leave an approval pending, so the background's
  20-second timer ran forever. Each file now stubs that timer, as two blocks
  in tests/backgroundApproval.test.js already did.
- Five popup test files show a flash message, and its timer outlived the
  test. They now run on fake timers.

Both test scripts in package.json drop --forceExit.

Model: opus-5-5

tests/addressKey.test.js

@@ -161,7 +161,7 @@ function click(element) {
 
 // show() starts the fee estimate without awaiting it; this lets it settle.
 async function settle() {
-    for (let i = 0; i < 10; i++) await new Promise((r) => setTimeout(r, 0));
+    for (let i = 0; i < 10; i++) await jest.advanceTimersByTimeAsync(0);
 }
 
 // ------------------------------------------------------------ harness
@@ -188,7 +188,11 @@ let sent;
 // handler to the buttons it drew, as the browser's fresh list would have.
 let plusButton;
 
+// showFlash() starts a timer that clears the message one or two seconds later.
+// Each test here runs on fake timers and drops them in afterEach, so that timer
+// never outlives the test.
 beforeEach(() => {
+    jest.useFakeTimers();
     approvalDetails = null;
     sent = [];
     mockNode.broadcast = [];
@@ -239,6 +243,7 @@ beforeEach(() => {
 });
 
 afterEach(() => {
+    jest.useRealTimers();
     jest.restoreAllMocks();
 });