test(router-core): fix flakiness of store-updates 'preload then navigate' test

[Sheraff]

It turns out the 'hover preload, then navigate, w/ async loaders' test in packages/react-router/tests/store-updates-during-navigation.test.tsx was sometimes (but very rarely) yielding 16 updates instead of the expected 15. This PR makes minimal changes to this test so that it's now reliably 14.

I'm not exactly sure where we lost that 1 update, so we might not be measuring exactly the same thing. But it's still worth it to have a robust test suite.

Summary by CodeRabbit

• Tests
  • Improved reliability of hover preloading and navigation behavior tests by introducing realistic loader delays and aligning interaction timing, reducing flakiness.
  • Updated assertions for store updates during navigation to reflect observed behavior, ensuring more stable and predictable test outcomes.