test: silence expected failure diagnostics

[s2x]

Description

Keep expected exception and warning diagnostics out of the PHPUnit progress output without changing production logging or error handling.

Changes

• Capture the expected ServerWorker backstop log in a temporary file.
• Use NullLogger for HttpRequestHandler tests that verify 500/error-path behavior rather than logging.
• Suppress only the expected mkdir(): Not a directory warning in the directory-failure test.
• Keep dedicated tests that verify error_log() fallback behavior unchanged.

Validation

• composer lint
• composer test

Changelog

No changelog entry: this is test-only and does not change library behavior.

Code Review

• Local diff review completed
• All local lint and test checks pass