Add test selector god-cli-flag

[Gankra]

We have logic to uniquely generate keys for a given TestKey, ensuring outputs have non-colliding names. As a corrollory you can technically refer to any combination of options with a really::long::list::of::options::like::this, but there's no way to provide that input or parse it. It would essentially be the reverse of this base_id, taking a string and producing a TestKey:

abi-cafe/src/harness/mod.rs

Lines 208 to 229 in 6ba6865

	impl TestHarness {
	pub fn base_id(
	&self,
	TestKey {
	test,
	options:
	TestOptions {
	convention,
	functions,
	val_writer,
	val_generator,
	},
	caller,
	callee,
	}: &TestKey,
	call_side: Option<CallSide>,
	separator: &str,
	) -> String {
	let mut output = format!("{test}{separator}{convention}");
	if let FunctionSelector::One { idx, args } = functions {
	let test = self.tests[test].clone();
	let func = test.types.realize_func(*idx);

This is mildly annoying to do because I decided options should be omitted if they're the default (for compactness), so you need to do a lot of "try to parse as WriteImpl, no, ok, what about..." to reverse the process. In principle it can be done though, and it would be very nice to be able to dump an id into the CLI and have it spit out exactly what you want!