Is it possible to conditionally exclude platforms from a target spec?

I have a framework target that is compiled for iOS, tvOS, and watchOS. I would like to create a unit test bundle for this framework and specify it in the target scheme definition. Here are the two targets in the spec:

PlatformDemoKit:
    type: framework
    platform: [iOS, tvOS, watchOS]
    sources:
        - PlatformDemoKit
    scheme:
        testTargets:
            - PlatformDemoKitTests_${platform}

PlatformDemoKitTests:
    type: bundle.unit-test
    platform: [iOS, tvOS]
    sources:
        - PlatformDemoKitTests
    dependencies:
        - target: PlatformDemoKit_${platform}

As you may have already guessed, this spec results in an error in XcodeGen:

Spec validation error: Target "PlatformDemoKit_watchOS" scheme has invalid test "PlatformDemoKitTests_watchOS"

Unfortunately, as of this writing, Xcode does not allow us to make test bundles with watchOS as the base SDK. Ideally, I would like to simply exclude the PlatformDemoKitTests_watchOS target from the testTargets list for watchOS given that it does not exist. Is what I'm trying to do possible in XcodeGen? Is there a more "idiomatic XcodeGen" way of solving this problem?

Start by tracing how XcodeGen expands platform-specific target names and validates scheme testTargets for the PlatformDemoKit example. Determine how a watchOS framework scheme can omit the unavailable test target while retaining it for iOS and tvOS, and verify the resulting generated project for all three platforms.