test(runtime-config): Add some tests to make sure data is cached

[john-z-yang]

Overview

We ran into an incident where we accidentally did not store runtime config into our cache and retrieved it from redis every time the get_runtime_config is called. This pr tries to add a couple of tests to assert the caching behaviour.

[volokluev]

Good start but what we really want to check is that get_str_config actually stores a value in cache if it retrieves it

[john-z-yang]

@volokluev I addedtest_runtime_config_is_stored_in_cache. Let me know how this looks

[john-z-yang]

I wanted to set this deadline to maybe 1 sec from now, and sleep for 1 sec to retrieve it, like so

#[test]
    fn test_runtime_config_invalidates_cache_outside_deadline() {
        {
            CONFIG.write().insert(
                "key3".to_string(),
                (
                    Some("value3".to_string()),
                    Deadline::new(Duration::from_secs(1)), // 1 instead of 0
                ),
            );
        }
        thread::sleep(Duration::from_secs(900));
        let config = get_str_config("key3");
        assert_eq!(config.unwrap(), None);
        CONFIG.write().clear();
    }

But for some reason this fails, and the debugger shows that coarsetime::Instant::recent() does not appear to be advancing