Add tests to ensure session is set correctly in OnChainScrapingVotes

[Lldenaurois]

Add assertions to runtime tests in order to ensure that session in OnChainScrapingVotes matches the expected session.

Addition to #4810

[drahnr]

A indirect test, but it covers it

[ordian]

I don't think it covers it. I've checked it before, it passed on master before #4810, because it triggers only else branch here

polkadot/runtime/parachains/src/paras_inherent/mod.rs

Lines 180 to 189 in 594e3b9

	if let Some(ref mut value) = value {
	value.backing_validators_per_candidate.clear();
	value.backing_validators_per_candidate.extend(backing_validators_per_candidate);
	} else {
	*value = Some(ScrapedOnChainVotes::<T::Hash> {
	backing_validators_per_candidate,
	disputes: MultiDisputeStatementSet::default(),
	session,
	});
	}

[drahnr]

Just stepped through with a debugger, @ordian is correct. I am working on a separate test case atm

[ordian]

why is it needed? isn't it used when cfg(test) is enabled?

[drahnr]

I saw an error in CI about unused imports, hence I added it. This will be refactored soon after #4834 since there is quite a bit of duplicate test code.