Consider relocating -Cpanic=abort logic

[kentfredric]

Currently there's this in src_configure:

https://github.com/gyakovlev/rust-dev-overlay/blob/5bde43d1f07f3cf55ab899ff394635a830ae182d/eclass/rust-crate.eclass#L138-L148

But that last part gets messed up if anybody does FEATURES=test USE=-test. That sounds odd, until you realize this is what USE=-* does :/

While of course we can say "Don't do that", we could also improve things if this flag wasn't defaulted to on globally. ( We could guard against this with RESTRICT="!test? ( test )", but doing that for hundreds of packages is pain ... especially when I'm using RESTRICT as the indicator I need to revisit the package to add tests in the first place :D )

This will especially matter when we're installing binary executables, we don't want the nature of those to be different simply because they were built by somebody who was wanting it tested.

Though the downside of that of course is, when we do ship binaries and build tests, the compile will have to do most of it twice, once for the binaries with -Cpanic=abort, and once for the tests ( without -Cpanic=abort ).