test/common/test_back_trace: skip one more frame when testing w/ ASan

test/common/test_back_trace: skip one more frame when testing w/ ASan

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.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>