test(integration-karma): add listener memoization tests

[nolanlawson]

Details

This adds Karma tests for event listener memoization, which came up when I was reviewing #4468.

This behavior is a little subtle, and previously we were only really testing it in the template-compiler snapshot tests here:

lwc/packages/@lwc/template-compiler/src/__tests__/fixtures/regression/handler-memoization/actual.html

Lines 4 to 9 in 39af4df

	<template for:each={list} for:item="task">
	<li key={task.id}>
	{task.title}
	<button onclick={task.delete}>[X]</button>
	</li>
	</template>

Adding the test ensures we don't regress the current functionality.

Does this pull request introduce a breaking change?

• 😮‍💨 No, it does not introduce a breaking change.

Does this pull request introduce an observable change?

• 🤞 No, it does not introduce an observable change.