test: add Bazel runfiles source aggregation

[longlho]

Summary

• Add a Bazel codescythe_test rule with a source aggregation provider/aspect.
• Add runfiles-style fixture packages, including a false-positive package.
• Make analysis roots absolute and follow symlinked runfiles directories/files.

Test plan

• bazel test --test_output=errors //tests/fixtures/runfiles-fixture:codescythe_runfiles_test //tests/fixtures/runfiles-false-positive:codescythe_false_positive_test
• cargo fmt --check
• cargo test --workspace
• bazel test //...