Add staging source-phase-imports

[legendecas]

Even though https://github.com/tc39/proposal-esm-phase-imports will supersede this test case, as https://tc39.es/proposal-source-phase-imports/#sec-source-text-module-record-getmodulesource is defined in the spec, add a staging test to cover import a SourceTextModule source.

Refs: #3980 (review)

[syg]

Wait, you don't need to manually add staging tests if you're working in V8. You can add these as part of the V8 CL and there's a 2-way sync bot that'll take care of the exporting. In that case you can run these tests as part of the V8 CQ.

[legendecas]

Yeah, I tried that. But the tools (and v8 test runner) complain about the folders that do not exist in test262 repo. So I manually picked the change here.

[ptomato]

There's no link to the V8 CL but if someone reviewed these there, I'm happy to click the merge button if for whatever reason the bot can't do it.

[syg]

There's no link to the V8 CL but if someone reviewed these there, I'm happy to click the merge button if for whatever reason the bot can't do it.

Please hold off for a little bit. I want to make sure whether there's a bug / unsupported use case with the 2-way sync bot.