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/deleteWalletLostPassword.test.js

@@ -102,6 +102,17 @@ function makeDocument() {
 
 // ------------------------------------------------------------ harness
 
+// showFlash() starts a timer that clears the message 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();
+});
+
+afterEach(() => {
+    jest.useRealTimers();
+});
+
 function wallet(name, secret, addresses) {
     return {
         type: "hd",