Enhance XCM Debugging with Log Capture in Unit Tests (#7594)

# Description

This PR introduces a lightweight log-capturing mechanism for XCM unit
tests, simplifying debugging by enabling structured log assertions. It
partially addresses #6119 and #6125, offering an optional way to verify
logs in tests while remaining unobtrusive in normal execution.

# Key Changes

* [x] Introduces a log capture utility in `sp_tracing`.
* [x] Adds XCM test examples demonstrating how and when to use log
capturing.

# Review Notes:

* The log capture mechanism is opt-in and does not affect existing tests
unless explicitly used.
* The implementation is minimal and does not add complexity to existing
test setups.
* It provides a structured alternative to
[`sp_tracing::init_for_tests()`](https://paritytech.github.io/polkadot-sdk/master/sp_tracing/fn.init_for_tests.html)
for log verification in automated tests.

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