Diem: remove custom_test_frameworks nightly feature

[bmwill]

The custom_test_frameworks nightly feature needs to be removed from the codebase so that we can migrate to using the stable rust compiler.

See #270 for more information.

[sunshowers]

I'd consider removing the feature nice to have but not blocking, as we always have the RUSTC_BOOTSTRAP workaround available for test code.

There are many affordances that custom_test_frameworks provides -- like allowing you to filter by filename on the CLI -- and it would be a shame to lose them.

[sunshowers]

https://github.com/commure/datatest/issues/4 has landed, so this should be possible now. Note that the switch to non-nightly datatest will have to happen concurrently with the switch to the stable release channel.

[bmwill]

Awesome! So the real blocker is #284 then. Also just to clarify, this should also work in the event we transition to beta next cycle?

[sunshowers]

Yeah, sorry about not being clear -- stable includes beta.