Previously, trace output (FD3) from skipped tests were not explicitly handled
in 'bats_tap_stream_comment()' function of 'bats-format-junit', so their trace
output fell into the catch-all '*' case:

  bats_tap_stream_comment() { # <comment text without leading '# '> <scope>
    local comment="$1" scope="$2"
    case "$scope" in
    begin)
      # everything that happens between begin and [not] ok is FD3 output from the test
      log_system_out "$comment"
      ;;
    ok)
      # non failed tests can produce FD3 output
      log_system_out "$comment"
      ;;
    *)
      # everything else is considered error output
      log "$1"
      ;;
    esac
  }

As a result, when using '--trace' flag, skipped tests output were incorrectly
reported as failures in the JUnit report because of this condition present in
'bats-format-junit':

  if [[ -n "$_buffer_log" || "$test_result_state" == not_ok ]]; then
    printf "        <failure type=\"failure\">%s</failure>\n" "$(xml_escape "${_buffer_log}")"
  fi

This change explicitly adds 'skipped' to the scope handled alongside 'ok',
so trace output for skipped tests goes to 'log_system_out "$comment"'. Skipped
tests are now correctly reported and not marked as failures.

Signed-off-by: Lucas Bernardes <[email protected]>