[Search Sessions] Improve functional tests due to a warning on fail to restore a search

[Dosant]

During work on #113950 there was a single test failure related to search sessions:

// SKIPPED - see https://github.com/elastic/kibana/pull/113950 
 // `expect(await toasts.getToastCount()).to.be(0)` fails because of a `Your search session is still running` toast 
 // rest of the test does pass 
 it.skip('relative timerange works', async () => { 

It turned out that the search session feature was broken at that point with failing to restore any searches. Most of the tests were passing, but only a single failure.

We need to review our tests so that in case the feature is degraded there is a more obvious indication of a problem:

During the initial feature, implementation searches were failed (and tests) in case of no search hash match during session restoration. But in #102057 we allowed searches to proceed with a warning in case no matches. With that change, we didn't properly update our tests to check if there are warnings that we don't expect.
As work on #113950 showed, the current test suite mostly passes in case searches can't be restored

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[kertal]

Closing this because it's not planned to be resolved in the foreseeable future. It will be tracked in our Icebox and will be re-opened if our priorities change. Feel free to re-open if you think it should be melted sooner.