Add test for difference between export * as x from and import * as x; export { x }

[nicolo-ribaudo]

This is a test for the difference described in tc39/ecma262#3710.

TL;DR

• export { foo } from "mod" and import { foo } from "mod"; export { foo } are equivalent: they both propagate the binding, so when both re-exported with export * from they do not cause ambiguity.
• export * as foo from "mod" and import * as foo from "mod"; export { foo } are not equivalent: the second one introduces a new binding, causing ambiguity. It would be unambiguous if both were doing export * as foo from "mod". It's still be ambiguous (two bindings) if both were doing import * as foo from "mod"; export { foo }

The first commit just moves the tests related to binding ambiguity to a single folder, so that it's easier to tell what is already tested.

[nicolo-ribaudo]

SM/JSC pass all the tests, while V8/engine262/XS fail test/language/module-code/ambiguous-export-bindings/namespace-unambiguous-if-export-start-as-from.js

[nicolo-ribaudo]

Marking as draft because I plan to present something related to this in plenary.

[guybedford]

Hmm, what is the value in allowing export * as foo to not be an ambiguous error? Is it not just an oversight that this doesn't throw as an ambiguous case? It doesn't seem to match the expectations for other unambiguous cases to me.