Resolve ros_distro selectors in package tests

[Tobias-Fischer]

• Resolve ros_distro selectors in package tests
• Read the test files as UTF-8, like the other text files since Read and write text files as UTF-8 #166

When a test file from tests/ is put into a recipe, if: selectors that test ros_distro are resolved. One test file can then serve several distributions, which matters once they share tests/ through extends. All other selectors (osx, win, …) are left unchanged for rattler-build.

tests:
  - script:
      - if: ros_distro in ["humble", "jazzy"]
        then:
          - if: osx
            then: export ROS_LOCALHOST_ONLY=1 && launch_test smoke_test.py
        else:
          - launch_test smoke_test.py

This replaces rendering distribution-specific tests with Jinja templates, as the distro template (Tobias-Fischer/ros-distro-template) did for robot_state_publisher. Tobias-Fischer/robostack-distros already uses these selectors in its shared tests for robot_state_publisher, ros2cli and rosidl_cli.

There's a new test for the resolution (vinca/test_distro_selectors.py); all 291 tests pass, ruff and pyrefly are clean, and the README has a short section.

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[Tobias-Fischer]

@traversaro this is ready for review/merge