Add .bzl files explicitly to toolchain tests to detect .bzl file changes

[fishcakez]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Python toolchain tests did not include the rules_python distribution directly in the tests, and so bazel could cache a toolchain test result even if the toolchain bzl files changed. This could mean that toolchain bugs could be introduced but CI would pass because the tests were cached and did not run.

Issue Number: #806

What is the new behavior?

Add the rules_python distribution to the test files/runfiles so that bazel can detect changes in the bzl files and invalidate the
test cache. Therefore CI should correctly catch test failures.

Does this PR introduce a breaking change?

• Yes
• No

Other information

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[fishcakez]

The CI failure is fixed in #805 and not directly related to this change, though the intent of this PR is to ensure tests/CI are behaving consistently.

[fishcakez]

@f0rmiga could you take a look as the author of these tests in #618. Currently folks can break toolchains and won't be caught in CI.