Testing "<packageName>PackgeTests" for multiple architectures triggers error message "Could not find target named"

[ahoppen]

Trying to run all tests in a package swift test --test-product <packageName>PackageTests fails if two architectures are passed.

Steps to reproduce:

$ mkdir /tmp/mypackage
$ cd /tmp/mypackage/
$ swift package init
$ swift test --test-product mypackagePackageTests --arch arm64 --arch x86_64
error: Could not find target named 'mypackagePackageTests_586B622B321A3B86_PackageProduct'
error: fatalError

I would have expected to build the tests for both architectures and run tests for the host architecture.

rdar://97876450