test: consistent fixture tests for disallowed @api props

[nolanlawson]

Details

This is extracted out of #4044 since it's not strictly related to that PR.

This just adds tests to confirm the behavior of disallowed @api properties defined here:

lwc/packages/@lwc/babel-plugin-component/src/constants.ts

Lines 23 to 26 in ccd0253

	// This set is for attributes that can never be defined
	// by users on their components.
	// We throw for these.
	const DISALLOWED_PROP_SET = new Set(['is', 'class', 'slot', 'style']);

Previously, only some of these props were tested. Now, they are all tested.

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.