sigusr2 autest: simplify Process and Ready objects

[bneradt]

Rewrite the sigusr2 autest to drive each scenario from a single
shel helper script instead of a chain of AuTest Processes.

This removes the Ready-condition races from the old test, keeps the
signal and log rotation ordering in one place, and makes the log file
expectations easier to follow. This should address flakiness in the
test that has been seen in CI.

[Copilot]

Pull request overview

This PR refactors the sigusr2 gold test to reduce flakiness by consolidating scenario orchestration into dedicated shell helper scripts instead of chaining multiple AuTest Process objects with Ready conditions.

Changes:

• Added sigusr2_rotate_diags.sh to perform diags.log rotation orchestration (wait → rename → SIGUSR2 → verify).
• Added sigusr2_rotate_custom_log.sh to drive configured access log rotation via requests + SIGUSR2 and verify log placement.
• Simplified sigusr2.test.py to use one default process per scenario that invokes the helper scripts, and moved log assertions into clearer per-scenario methods.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/gold_tests/logging/sigusr2_rotate_diags.sh	New helper script that deterministically sequences diags.log rename + SIGUSR2 + content checks.
tests/gold_tests/logging/sigusr2_rotate_custom_log.sh	New helper script that drives /first→rotate→/second→SIGUSR2→/third and waits for expected log writes.
tests/gold_tests/logging/sigusr2.test.py	Reworked test to call the helper scripts (single-process orchestration per run) and to simplify ATS/server setup and assertions.

[bryancall]

The shell-script approach is a big improvement over the old Process chain. Much easier to follow the sequencing, and the explicit polling loops with 60-second timeouts should handle the ASAN/CI slowness that was causing the flakiness.

Two things I noticed:

1. Missing StillRunningAfter -- The old test had tr1.StillRunningAfter = diags_test.ts (and the server) to verify ATS survived the signal. Neither add_system_log_test nor add_configured_log_test sets this. Without it, if ATS crashes during SIGUSR2 handling, the test could still pass as long as the shell script exits 0 (the shell script's wait_for_contains would succeed on files written before the crash). Might be worth adding back.

2. Typo in commit message -- "shel helper script" should be "shell helper script" (appears twice).