Fix test_shim_integration: write aliases to /etc/bash.bashrc by setting remoteUser to root

[Copilot]

mcr.microsoft.com/devcontainers/base:ubuntu advertises vscode as its default remote user via image metadata. Without an explicit remoteUser, the devcontainer CLI infers _REMOTE_USER=vscode, causing install.sh to set INSTALL_WITH_SUDO=true and write shell function aliases to /home/vscode/.bashrc instead of /etc/bash.bashrc. All six alias-related assertions in test_shim_integration.sh check /etc/bash.bashrc, so they all fail.

Changes

• test/artifacts-helper/scenarios.json: Add "remoteUser": "root" to the test_shim_integration scenario, ensuring _REMOTE_USER=root → INSTALL_WITH_SUDO=false → aliases written to /etc/bash.bashrc.

"test_shim_integration": {
    "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
    "remoteUser": "root",   // <-- added
    "features": { ... }
}

This mirrors the existing pattern: test_non_root_install explicitly sets "remoteUser": "vscode" to test the non-root path; test_shim_integration should explicitly set "remoteUser": "root" to test the root/system-wide path.