ch_tests_tool: parse nextest JUnit output for cloud-hypervisor tests

[anirudhrb]

Description

The cloud-hypervisor integration suite now emits per-suite JUnit XML via cargo nextest. Prefer that structured output over the legacy cargo stdout scraping so subtest results are accurate and resilient to log-format churn upstream.

• Collect JUnit XMLs from the SSH user's $HOME/workloads/junit/, which is where dev_cli.sh's docker container (run with --user $(id -u):$(id -g) and bind-mounting $HOME/workloads) writes them as the SSH user.
• Resolve $HOME on the remote node before each use, falling back to /root/workloads/junit if resolution fails.
• Wipe any stale JUnit dir from a previous run on the same node before starting tests so results can never be misattributed across runs.
• Parse elements into CloudHypervisorTestResult, mapping failure/error -> FAILED, skipped -> SKIPPED, otherwise PASSED. Subtests expected but missing from every JUnit file stay QUEUED. Last retry wins, matching nextest's final-outcome reporting.
• When 'cargo nextest' appears in stdout but no JUnit files are produced, fail with a clear diagnostic instead of silently falling back to the stdout parser.
• Fall back to the existing stdout parser when no JUnit files are present and nextest wasn't invoked (classic cargo test path).

Related Issue

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Documentation update

Checklist

• Description is filled in above
• No credentials, secrets, or internal details are included
• Peer review requested (if not, add required peer reviewers after raising PR)
• Tests executed and results posted below

Test Validation

Key Test Cases:

Impacted LISA Features:

Tested Azure Marketplace Images:

Test Results

Image	VM Size	Result
		PASSED / FAILED / SKIPPED

[Copilot]

Pull request overview

This PR updates the Cloud Hypervisor test runner tool to prefer parsing cargo nextest JUnit XML output (copied back from the VM) over the legacy stdout scraping, improving subtest result fidelity and resilience to upstream log-format changes.

Changes:

• Collect nextest-generated JUnit XML files from the remote node and parse <testcase> elements into CloudHypervisorTestResult statuses.
• Add a guardrail to fail with a clear diagnostic when nextest appears to have run but no JUnit output is found.
• Proactively remove any stale remote JUnit output directory before running tests to avoid misattributing results across runs.