Switch test fixtures to EVGEN-relative paths under /volatile/eic/EPIC/EVGEN

[Copilot]

This updates the test suite to consistently target EVGEN data using server-relative paths rooted at /volatile/eic/EPIC/EVGEN. It removes mixed RECO/legacy absolute path assumptions that could resolve outside the intended test base.

• Base directory defaults

  • test/xrootd.test.ts: default XROOTD_BASE_DIR changed to /volatile/eic/EPIC/EVGEN.
  • test/root-analysis.test.ts: XRootDClient base dir updated from /work/eic2/EPIC to /volatile/eic/EPIC/EVGEN.

• Path normalization to EVGEN-relative usage

  • Converted hardcoded file/dataset test inputs to EVGEN-relative paths (e.g., SIDIS/...) instead of absolute/base-prefixed paths.
  • Updated large-directory scenario from RECO-specific path to EVGEN path (SIDIS).

• Fixture naming alignment

  • Replaced RECO-style default sample file names (*.eicrecon.tree.edm4eic.root) with EVGEN-style names (*.hepmc3.tree.root) in test defaults/examples where applicable.

• Test docs updates

  • test/README.md now documents EVGEN as the default base path and updates troubleshooting xrdfs examples accordingly, while keeping custom path examples generic.

// before
const TEST_BASE_DIR = process.env.XROOTD_BASE_DIR || '/volatile/eic/EPIC';
arguments: { path: `${TEST_BASE_DIR}/nonexistent-test-file.root` }

// after
const TEST_BASE_DIR = process.env.XROOTD_BASE_DIR || '/volatile/eic/EPIC/EVGEN';
arguments: { path: 'nonexistent-test-file.root' }