Use public rules_testing APIs once available

PR #1287 introduces several usages of private APIs and yet-to-be-released APIs in rules_testing.

This issue is to track updating the rules_python code to use those apis once they are public and released in rules_testing.

* [ ] tests/cc/py_cc_toolchain/py_cc_toolchain_tests.bzl -- Use DictSubject.get once available (https://github.com/bazelbuild/rules_testing/issues/51)
* [ ] tests/default_info_subject.bzl -- Use runfiles subject once available (https://github.com/bazelbuild/rules_testing/issues/54)
* [ ] tests/default_info_subject.bzl -- use default info subject once available (https://github.com/bazelbuild/rules_testing/issues/52)
* [ ] tests/py_cc_toolchain_info_subject.bzl -- Use dict subject once available (https://github.com/bazelbuild/rules_testing/issues/54)
* [ ] tests/py_cc_toolchain_info_subject.bzl -- Use str subject once available (https://github.com/bazelbuild/rules_testing/issues/54)
* [ ] tests/struct_subject.bzl -- use struct subject once available (https://github.com/bazelbuild/rules_testing/issues/53)

Empieza comprobando los issues 51–54 de rules_testing y su estado de publicación; después, inspecciona los archivos .bzl enumerados para identificar los usos actuales de APIs privadas o no publicadas. Actualiza cada uso cuando las APIs públicas correspondientes estén disponibles y verifica que las pruebas afectadas de rules_python pasen.

Escrito por el modelo de indexación a partir del texto del issue.