…st test

The gazelle_python_manifest test was failing on Linux but passing on macOS
due to incorrect file path handling. The test used $(rootpath) instead of
$(rlocationpath) for data files and didn't use the resolved runfiles paths
returned by runfiles.Rlocation().

Changes:
- Update defs.bzl to use $(rlocationpath) for _TEST_MANIFEST and
  _TEST_REQUIREMENTS environment variables (matching the existing pattern
  for _TEST_MANIFEST_GENERATOR_HASH)
- Fix test.go to use the resolved paths from runfiles.Rlocation() instead
  of the unresolved paths at lines 53, 73, and 84-86

This ensures the test works consistently across all platforms by properly
using the Bazel runfiles mechanism.

Fixes #3397

Note: Some code generation was assisted by Claude AI. The human author has
reviewed, tested, and takes full responsibility for all changes.

Address code review feedback from Gemini Code Assist bot by creating a
getResolvedRunfile helper function. This reduces code duplication and
improves error messages by consolidating the logic for:
- Getting environment variable values
- Validating they are set
- Resolving paths through runfiles.Rlocation()

Applied the helper to all three environment variables:
- _TEST_REQUIREMENTS
- _TEST_MANIFEST
- _TEST_MANIFEST_GENERATOR_HASH

This reduces ~20 lines of duplicated code to 3 function calls and
provides more specific error messages that include the env var name
and unresolved path.

Addresses: #3398 (comment)

…-plugin-requirements-data-paths