test: capture DUT syslog per test

[saba8814]

Description

This PR introduces a way for Infamy to collect each DUT's syslog per test.
Each DUT sends its syslog to the test container, which saves it per test
and DUT next to the test output:

test/.log/<run>/output/0006-set-hostname.log
test/.log/<run>/syslog/0006-set-hostname/target/{syslog,kern.log,messages}

After the run, make test packs the logs in logs.tar.gz, which CI uploads
as the dut-syslog artifact. Capture is off by default, enable it with
make TEST_SYSLOG_CAPTURE=y test.

The start and stop of each test, and each test step, are marked in the
DUT's log with the host's time, so DUT lines can be lined up with the test
output. Capture is best-effort: if a DUT reboots, its log is kept up to
the reboot and resumes when the test attaches again. Tests that set up
syslog themselves stop the capture for that DUT. The test result is
never affected.

Resolves: #1608

Checklist

Tick relevant boxes, this PR is-a or has-a:

• Bugfix
  • Regression tests
  • ChangeLog updates (for next release)
• Feature
  • YANG model change => revision updated?
  • Regression tests added?
  • ChangeLog updates (for next release)
  • Documentation added?
• Test changes
  • Checked in changed Readme.adoc (make test-spec)
  • Added new test to group Readme.adoc and yaml file
• Code style update (formatting, renaming)
• Refactoring (please detail in commit messages)
• Build related changes
• Documentation content changes
  • ChangeLog updated (for major changes)
• Other (please describe):