Simplify U-mode validation test output format

[HeatCrab]

The previous test output used phase-based grouping and hierarchical numbering which was inconsistent with other test apps in the codebase. This change aligns with the sequential numbering convention used elsewhere and improves readability with descriptive progress messages.

Related to #62.

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Summary by cubic

Standardizes the U-mode validation test output to a simple, sequential format with clear PASS/FAIL markers and short progress messages. Aligns with other test apps and the standardization goals in Linear #62.

• Refactors
  • Replaces phase-based, hierarchical numbering with Tests 1–4.
  • Adds concise progress lines (e.g., "Calling sys_tid()", "Setting SP to 0xDEADBEEF").
  • Normalizes result lines to “[PASS]/[FAIL]” with returned values.
  • Simplifies privileged CSR check messaging and clarifies failure condition.

^{Written for commit 1f899ce. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

[jserv]

Thank @HeatCrab for contributing!