[infra] Enable skipping tests via environment variables

We should add some logic to our test suites so that we can invoke them to only run all tests targeting a specific architecture/os.

Use cases:

* Locally running tests if not all tooling is installed.
* Running extra CI jobs targeting different compilers, and not having to install the compilers for all the other architectures/oses. https://github.com/dart-lang/native/pull/135