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/chainSwitchGate.test.js

@@ -56,7 +56,18 @@ async function settle() {
     for (let i = 0; i < 50; i++) await Promise.resolve();
 }
 
+// While an approval is pending, the background starts a 20-second timer to
+// keep itself running. The timer is stubbed and never starts, so no test in
+// this file leaves it running.
+let keepRunningTimer;
+beforeEach(() => {
+    keepRunningTimer = jest
+        .spyOn(global, "setInterval")
+        .mockImplementation(() => 1);
+});
+
 afterEach(() => {
+    keepRunningTimer.mockRestore();
     delete global.chrome;
 });