Create test case to prevent nested selectors

[Pike]

We should audit the test coverage for

fluent/syntax/abstract.js

Lines 54 to 61 in 94dae4d

	return ([selector, variants]) => {
	let selector_is_valid =
	selector instanceof FTL.StringLiteral
	|| selector instanceof FTL.NumberLiteral
	|| selector instanceof FTL.VariableReference
	|| selector instanceof FTL.FunctionReference
	|| (selector instanceof FTL.TermReference
	&& selector.attribute);

Notably, I don't think we have a test to check for select expressions as selectors.

AFAICT, this also bleeds into our implementations.