Refine test_on_native_only option in case of multiple cross built platforms

[mbargull]

At conda-forge/rust-feedstock#75 we had the (potential) case of cross building for multiple platforms for which only some could be tested via emulation.
The feedstock uses test_on_native_only: true because it builds for osx-arm64 on osx-64. If we cross-compiled some Linux platforms, we could make use of QEMU-based tests, but this is prevented due to test_on_native_only: true.

We might want a more fine grained configuration, either per target platform or per platform type.
Suggestions thus far (@mbargull/@isuruf in conda-forge/rust-feedstock#75 (comment) ff):

Would it make sense to make test_on_native_only optionally a dict platform -> bool to distinguish?

Yes. I think we can also have an option like,

test: <native-only/all/native-and-emulated>

and deprecate test_on_native_only option. What do you think?

[leofang]

Pointed to here by @chrisburr on Gitter. I am evaluating the very same strategy for Open MPI, so that we can move away from Travis and use Azure to build the ppc64le package, yet still being able to run tests via QEMU (no idea how it works ftm).