Merge branch 'sg/httpd-test-unflake'

httpd tests saw occasional breakage due to the way its access log
gets inspected by the tests, which has been updated to make them
less flaky.

* sg/httpd-test-unflake:
t/lib-httpd: avoid occasional failures when checking access.log
t/lib-httpd: add the strip_access_log() helper function
t5541: clean up truncating access log