test,tools: move Flags: comment processing from Python to JavaScript

[Trott]

This is a big change set but one that can be broken down into many smaller pull requests for easier review if necessary.

This adds common.relaunchWithFlags() along with two useful-but-not-strictly-necessary helper functions, common.exposeInternals() and common.experimentalWorker().

This allows tests to relaunch themselves with the appropriate command line flags.

Advantages:

• Tests work right from the command line without having to specify the flags or use the Python test runner.
• Less magic. Having what is going on be presented explicitly in the executable JavaScript is preferable to comments affecting code but only affecting it when run a certain way.
• Related to the previous two, but this is more friendly to new contributors and sporadic/infrequent contributors.
• This removes a small bit of dependency on Python that isn't necessary. Arguably, this aligns with Suggestion: New Python & GYP strategic initiative or working group TSC#642.

Downside:

• It takes slightly longer for these tests to run. Not much, though. On my laptop, make test with this changeset took 5 minutes and 23 seconds. On master, it took 5 minutes and 16 seconds.

Upside to the downside:

• If this encourages us to go through and figure out places where tests can be appropriately and correctly refactored to not use CLI flags, that's a win. In many cases, the use of a flag (especially --expose-internals) could be a sign the test is testing the wrong thing.

@nodejs/testing

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines