Claude review: kunit: Add backtrace suppression self-tests

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Good test coverage.** The tests exercise:
- Direct WARN() suppression with count verification
- Indirect WARN() from a called function (marked `noinline` — good, prevents inlining from changing the warning's call site)
- Multiple warnings in one block
- WARN_ON() variants (with appropriate CONFIG_DEBUG_BUGVERBOSE/CONFIG_KALLSYMS skip conditions)
- Incremental counting
- Active-state transitions around suppression boundaries
- Multi-scope using the direct API with sequential independent suppression blocks
- Cross-kthread isolation

**The multi_scope test** validates that after `kunit_end_suppress_warning(test, sw1)`, the handle `sw1` still has valid count data. This works because `kunit_release_action()` removes it from the list and frees the action entry, but the `kunit_kzalloc`'d memory is still live until test teardown. This is subtle but correct — worth noting that `sw1`'s memory outlives the suppression because it's managed by `kunit_kzalloc()` (test-lifetime), not the action.

**Minor nit:** The `backtrace_suppression_test_warn_on_direct` test skips if `!CONFIG_DEBUG_BUGVERBOSE && !CONFIG_KALLSYMS`, while `backtrace_suppression_test_warn_on_indirect` only checks `!CONFIG_DEBUG_BUGVERBOSE`. The asymmetry is because direct WARN_ON() at the call site can use KALLSYMS to resolve the address, but indirect WARN_ON() from a noinline helper needs BUGVERBOSE for file/line info. This seems intentional.

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Generated by Claude Code Patch Reviewer