Don't load plugins synchronously in tests

[nicolo-ribaudo]

Q	A
Tests Added + Pass?	Yes
License	MIT

Ref #13414. When migrating to ESM, plugins must either be pre-loaded before calling transformSync or you must use transformAsync so that Babel can internally use dynamic import.

In most cases this is fine because Babel already runs asynchronously, but there were a few places in our tests that needed to be updated. Note that this is the biggest breaking change of the ESM migration, but:

• most Babel users don't use Babel directly, and all the popular integrations already call it asynchronously
• users can either choose to migrate to ESM and hoist plugins to normal import declarations, or they can call Babel asynchronously: since Babel is used to transform files, usually it happens already where there is other i/o (which is often async)
• if we do Move stable ECMAScript plugins to @babel/core rfcs#10, we could mitigate this change by making sure that at least @babel/preset-env can be used synchronously if necessary

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/52311/

[JLHwung]

Q: why do we need returning the promise?

[nicolo-ribaudo]

Otherwise the test runner cannot know if it was resolved successfully or rejected.

[JLHwung]

Can we move the runCodeInThisContext wrapper

babel/packages/babel-helper-transform-fixture-test-runner/src/index.ts

Line 228 in b1e73d6

const src = `(function(exports, require, module, __filename, __dirname, opts) {\n${code}\n});`;

before the Babel transform?

babel/packages/babel-helper-transform-fixture-test-runner/src/index.ts

Line 302 in b1e73d6

babel.transformAsync(execCode, execOpts),

So that we don't have to change test options and return in exec.js is automatically valid.

[nicolo-ribaudo]

That's a good idea, but there are manylre tests that return a promise and thus I'd prefer to do it as a separate PR.