fix(test): check for zebrad test output in the correct order

[teor2345]

Motivation

The sync_until function was checking for mempool activation in unreliable and slow ways:

1. The sync test was checking for mempool output after the stop log, but sometimes the mempool activates before the stop log
2. The non-mempool tests were checking each individual log line, but they need to check the entire log

Solution

1. The mempool is only activated once, so we must check for that log first. After mempool activation, the stop regex is always logged at least once. (It might be logged before mempool activation as well, but we can't rely on that.)

2. When checking that the mempool didn't activate, wait for the zebrad command to exit, then check the entire log.

Review

@jvff can review this PR, but I'm going to cherry-pick it into #3582 as well.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

Follow Up Work

• Run full sync tests on Mainnet #1592

[codecov]

Codecov Report

Merging #3643 (190cee2) into main (397ba1f) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3643      +/-   ##
==========================================
- Coverage   79.95%   79.89%   -0.06%     
==========================================
  Files         281      281              
  Lines       32584    32584              
==========================================
- Hits        26052    26034      -18     
- Misses       6532     6550      +18     

[jvff]

Looks great! Simple and elegant solution 🎉