Limit tests to platforms supported by a package by default

[felixsinger]

Issue description

I've noticed in various situations that the CI tries to execute tests on platforms which are not supported by the package. That leads to unnecessary evaluation attempts and ultimately errors in the CI summary of a pull requests. One of my recent examples is Redmine.

However, I've just learned that tests may be limited to specific platforms with handleTestOn in nixos/tests/all-tests.nix. By using that, it seems, the CI won't even try to run the tests on an unsupported platform and the unsupported platform is just marked as successful in the summary.

Now, I'm wondering why handleTest doesn't use the platforms supported by a package as default. For example, without the limitation I've done in #348178 the CI will try to test Redmine on Darwin platforms even if they are not supported by the Redmine package.