tests: fix readonly variables error with bash 4.4

Local variables are always visible to child functions, when they are
declared readonly and the same variable name is reused this results in
an error with bash 4.4.

As a workaround, rename the affected variables to fix the following
errors:

    ./tests/utils/utils.sh: line 222: args: readonly variable
    ./tests/utils/utils.sh: line 303: extra_details_args: readonly variable

Change-Id: I7321a9940a2e69bede363acddbd5f0465a3b1433
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12000
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>