tests: fix shellcheck errors in plugins/sink.text.pretty/test_enum

tests: fix shellcheck errors in plugins/sink.text.pretty/test_enum

Fix:

    In test_enum line 22:
    temp_stderr_expected="/dev/null"
    ^------------------^ SC2034 (warning): temp_stderr_expected appears unused. Verify use (or export if used externally).

    In test_enum line 40:
                    "$(cat $expected_file)" "$(cat $actual_file)"
                           ^------------^ SC2086 (info): Double quote to prevent globbing and word splitting.
                                                   ^----------^ SC2086 (info): Double quote to prevent globbing and word splitting.

    Did you mean:
                    "$(cat "$expected_file")" "$(cat "$actual_file")"

    In test_enum line 49:
            local test_text=
                  ^-------^ SC2034 (warning): test_text appears unused. Verify use (or export if used externally).

    In test_enum line 80:
            if (($expected_to_fail)); then
                 ^---------------^ SC2004 (style): $/${} is unnecessary on arithmetic variables.

Change-Id: I1956a395dd8638b5e7ee9d4a91301243a6ad76ee
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8636
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>