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

Some tests left timers running when they ended, and --forceExit hid them; both test scripts drop it. Six background test files left the background's 20-second timer running while an approval was pending; each now stubs it for the whole file. Five popup test files left a flash message's two-second timer running; they now use fake timers, and a wait on real zero-delay timeouts became jest.advanceTimersByTimeAsync(0). Two ethers timers can still run for up to 250 ms after a test ends; they stop before jest checks for open handles.

Model: opus-5-5

tests/walletDefects.test.js

@@ -357,6 +357,11 @@ describe("every way to a signature or the private key refuses a defective wallet
     });
 
     beforeEach(() => {
+        // 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.
+        jest.useFakeTimers();
+
         const broken = brokenXprvWallet();
         // A balance, so that no Send button's zero-balance refusal can stand
         // in for the defect check.
@@ -398,6 +403,7 @@ describe("every way to a signature or the private key refuses a defective wallet
     });
 
     afterEach(() => {
+        jest.useRealTimers();
         state.wallets = [];
         state.activeAddress = null;
         state.selectedWallet = null;