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)

Start by checking rules_testing issues 51–54 and their release status, then inspect the listed .bzl files for the current private or unreleased API usages. Update each usage once the corresponding public APIs are available, and verify the affected rules_python tests pass.

Written by the indexing model from the issue text.