tests: utils.sh six stacktrace

tests: utils.sh six stacktrace

Changes:

- BASH_SOURCE index was one off.
- The first line of stacktrace was pure confusion displaying executed
  script together with innermost line number (which was either 125 when
  STACKTRACE or 229 when skip was called.)
- We can safely ignore innermost call, as stack trace is always produced
  by stacktrace function.
- It is safer to test for array length, instead of testing FUNCNAME is
  main - if main function were introduced.
- Bashishm is safe to use as this function as a whole is relying on bash.