tests(smoke): add expectation for lcp-element

[connorjclark]

Smokerider did not catch the error from #15075 because we have no smoke tests that check largest-contentful-paint-element with simulated throttling.

[adamraine]

Wouldn't the TTFB error be caught here:

lighthouse/cli/test/smokehouse/test-definitions/dobetterweb.js

Lines 588 to 597 in 469157a

	'metrics': {
	// Flaky in DevTools
	_excludeRunner: 'devtools',
	details: {items: {0: {
	timeToFirstByte: '450+/-100',
	lcpLoadStart: '7750+/-500',
	lcpLoadEnd: '7750+/-500',
	}}},
	},
	},

[connorjclark]

yeah. also tho we aren't running dbw in smokerider yet, until #15111

won't hurt to also add a check for lcp element audit