tests: don't fail tests on torn log lines (#5655)

## Problem

Tests that force-kill and restart a service can generate torn log lines
that might match WARN|ERROR, but not match the allow expression that a
test has loaded, e.g.
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-5651/6638398772/index.html#suites/7538959189f4501983ddd9e167836c8b/d272ba8a73e6945c

## Summary of changes

Ignore log lines which match a regex for torn log lines on restart: they
have two timestamps and the second line is an "INFO version"... message.

test_runner/fixtures/neon_fixtures.py

@@ -1719,6 +1719,11 @@ class NeonPageserver(PgProtocol):
                 break
 
             if error_or_warn.search(line):
+                # Is this a torn log line?  This happens when force-killing a process and restarting
+                # Example: "2023-10-25T09:38:31.752314Z  WARN deletion executo2023-10-25T09:38:31.875947Z  INFO version: git-env:0f9452f76e8ccdfc88291bccb3f53e3016f40192"
+                if re.match("\\d{4}-\\d{2}-\\d{2}T.+\\d{4}-\\d{2}-\\d{2}T.+INFO version.+", line):
+                    continue
+
                 # It's an ERROR or WARN. Is it in the allow-list?
                 for a in self.allowed_errors:
                     if re.match(a, line):