Description

General improvements as noted below, makes the test more robust for any future additions.

Revisions:

• Removed some outdated comments about wrappers / timings that are no longer relevant.
• pgrep check now uses --full which was initially required for MTA-STS support to pass the test, whilst now it is more useful for matching postfix/amavis better.
• pkill now kills the process and asserts against the oldest matched process PID from pgrep. This should be the process that spawned any relevant children. Matching against the PID is better for targeting what we're interested in, since the process path is not always consistent (/usr/sbin/amavi vs /usr/sbin/amavisd-new).
• Added context for the python3 and postgrey services, along with additional maintainance comments on pkill / pgrep usage.

Type of change

• Improvement (non-breaking change that does improve existing functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes
• I have added information about changes made in this PR to CHANGELOG.md