Find a way to support iOS 9.3 tests on Catalina

[jverkoey]

Catalina does not support running tests on iOS 9.3 devices due to the fact that it no longer supports 32 bit. So long as we are supporting iOS 9, we'd like to still be able to run tests (likely as part of CI), but we also want to make it easy to run bazel test //... locally.

This may be some combination of select statements and https://docs.bazel.build/versions/master/platforms.html to conditionally include an iOS 9.3 simulator on Catalina.

[romoore]

We could postpone Catalina updates until we've dropped iOS 9 support. I believe we're pushing hard to drop it and this is yet another reason to do so (lack of Catalina support).

[jverkoey]

I'm not certain we can retroactively ask folks to downgrade from Catalina if that's already happened (e.g. my machine is now running Catalina).

[romoore]

Since MDC-iOS currently only supports Xcode 10, does it make sense to require Catalina yet?

[jverkoey]

We are not presently requiring Catalina.

[romoore]

So we could keep things running on Xcode 10 with iOS 9, yes? Recommending Mojave to contributors who want to run tests locally?

[jverkoey]

The purpose of this issue is to try to support both Catalina and Mojave for local development.

[romoore]

If Catalina doesn't support iOS 9 simulators but MDC-iOS has to support iOS 9 device/simulator testing, then it seems like Catalina isn't possible for MDC-iOS (at least). Of course I know that doesn't stop MDF libraries from dropping iOS 9, but it seems like this library is primarily supporting MDC-iOS at the moment.

[jverkoey]

And the resolution of this issue should be that both Catalina and Mojave are supported. iOS 9 testing will be done on CI at a mimimum. If someone is on Catalina then they will need to rely on CI to test iOS 9.