if ASan is enabled, backtrace() returns one more frame, so we need to skip one more frame when testing backtrace facility. this change addresses the following test failure:

[ RUN      ] BackTrace.Basic
/home/jenkins-build/build/workspace/ceph-pull-requests/src/test/common/test_back_trace.cc:43: Failure
Value of: std::regex_match(lines[lineno], e)
  Actual: false
Expected: true
[  FAILED  ] BackTrace.Basic (5 ms)

it only manifests itself when testing with ASan enabled.

Contribution Guidelines

• To sign and title your commits, please refer to Submitting Patches to Ceph.

• If you are submitting a fix for a stable branch (e.g. "quincy"), please refer to Submitting Patches to Ceph - Backports for the proper workflow.

• When filling out the below checklist, you may click boxes directly in the GitHub web UI. When entering or editing the entire PR message in the GitHub web UI editor, you may also select a checklist item by adding an x between the brackets: [x]. Spaces and capitalization matter when checking off items this way.

Checklist

• Tracker (select at least one)
  • References tracker ticket
  • Very recent bug; references commit where it was introduced
  • New feature (ticket optional)
  • Doc update (no ticket needed)
  • Code cleanup (no ticket needed)
• Component impact
  • Affects Dashboard, opened tracker ticket
  • Affects Orchestrator, opened tracker ticket
  • No impact that needs to be tracked
• Documentation (select at least one)
  • Updates relevant documentation
  • No doc update is appropriate
• Tests (select at least one)
  • Includes unit test(s)
  • Includes integration test(s)
  • Includes bug reproducer
  • No tests