libtest --include-ignored nightly-gating is confusing

[ehuss]

The libtest --include-ignored flag is intended to be unstable, but it does not present a clear way to use it.

> cargo +nightly test -- --include-ignored
error: The "include-ignored" flag is only accepted on the nightly compiler
error: test failed, to rerun pass '--lib'

Hmm, I'm running on nightly. Only through some sleuthing can one find out that you have to pass -Zunstable-options to make it work:

cargo +nightly test -- --include-ignored -Zunstable-options

There are several issues here (some of which were noted in the original PR):

• The text "is only accepted on the nightly compiler" should probably say something about -Zunstable-options.
• The text "is only accepted on the nightly compiler" is wrong, it works on stable, too. And this isn't a compiler.
• There is no tracking issue for this feature, so there is nowhere to discuss it. Should one be created?
• The --help text gives no indication that this flag is unstable.
• Is there any reason not to just stabilize it now?

[gilescope]

I think it's a footgun that cargo test -- --ignored is the only stable option. We should eliminate that footgun by stabilising --include-ignored.

How does one nominate for stabilising something?