End-to-end tests lack coverage for deducting impression site quotas

[apasel422]

All the current tests still pass if this block is commented out:

attribution/impl/src/backend.ts

Lines 576 to 590 in c18f38a

	for (const impression of impressions) {
	const impressionSite = impression.impressionSite;
	const impressionQuotaKey = { site: impressionSite, epoch };
	const entryI = getOrInsertEntry(
	this.#impressionSiteQuotaStore,
	impressionQuotaKey,
	this.#delegate.impressionSiteQuotaPerEpoch,
	);
	if (
	getEntry(deductedImpressionQuotas, impressionQuotaKey) === undefined
	) {
	deductedImpressionQuotas.push(impressionQuotaKey);
	entryI.value -= valueDeduction;
	}
	}

[bmcase]

@apasel422 thanks for flagging; I'll work on adding some additional tests on this