add test_type to TestDesc to compile on nightly rust

[cormacrelf]

Not sure if this should be more complicated, e.g.:

• use if_rust_version to do what the unimplemented cfgs would do and
  enable test_type for those 6-day-old nightlies
• add macro syntax for changing the test type? (probably not necessary)
• update docs to reflect {usability with stable rust} not being itself stable?

Also obviously this is going to fail most tests

[cormacrelf]

Ok, so:

• the new parse_opts location also needs papering over in the post_v139 feature
• CI needs the feature enabled

It seems like a lot of work to paper over all the libtest API we use to keep the stable guarantee... what other changes is libtest undergoing? Are there going to be changes that will break the ability to provide the facade?

[idubrov]

Yeah... I've been waiting for rust-lang/rust#50297 to happen, which would, hopefully, solve these woes.

[idubrov]

I think, clippy runs on stable so maybe it doesn't need that feature set?

[cormacrelf]

No, the clippy task is running on nightly.

[idubrov]

Thanks for doing the work!