Improve notify_message output to not interpret % as placeholder

[aarvio]

• Use similar printf formating for both text and TAP formats. Prevent printf from interpreting % as format placeholders and backslash escape sequences, preserving exact failure messages.
• Add shellcheck disable 2329 (same reasoning as 2317).

This ensures notify_message prints messages literally, avoiding misleading or mangled failure output.

[pgrange]

Thanks @aarvio for your contribution but there's a problem. bash_unit depends on printf formatting in several places.

For instance, look into assert_equals:

assert_equals() {
  local expected=$1
  local actual=$2
  local message=${3:-}
  [[ -z $message ]] || message="$message\n"

  if [ "$expected" != "$actual" ]
  then
    fail "$message expected [$expected] but was [$actual]"

See how \n is added at the end of the message so that the user message is displayed on a dedicated line and then the inequality issue on another line.

Looking at that, this is ugly but it needs to be addressed before accepting your contribution. I'm not sure how, though 🤔

[aarvio]

Maybe clearer just to ensure that % is not interpreted as placeholder and keep the escape sequence rendering as it is.

[aarvio]

@pgrange updated the topic of PR to match the content.

Escape sequences are handled as before but % should not be interpreted as placeholder.
Makes the printf format same in basic output as it already is with tap_format.