tests: declare and assign variables separately in test_exit_status

Fixes these shellcheck errors:

    In test_exit_status line 34:
            local actual_stdout=$(mktemp -t test_cli_exit_status_stdout_actual.XXXXXX)
                  ^-----------^ SC2155: Declare and assign separately to avoid masking return values.

    In test_exit_status line 35:
            local actual_stderr=$(mktemp -t test_cli_exit_status_stderr_actual.XXXXXX)
                  ^-----------^ SC2155: Declare and assign separately to avoid masking return values.

    In test_exit_status line 53:
            local actual_stdout=$(mktemp -t test_cli_exit_status_stdout_actual.XXXXXX)
                  ^-----------^ SC2155: Declare and assign separately to avoid masking return values.

    In test_exit_status line 54:
            local actual_stderr=$(mktemp -t test_cli_exit_status_stderr_actual.XXXXXX)
                  ^-----------^ SC2155: Declare and assign separately to avoid masking return values.

    In test_exit_status line 72:
            local actual_stdout=$(mktemp -t test_cli_exit_status_stdout_actual.XXXXXX)
                  ^-----------^ SC2155: Declare and assign separately to avoid masking return values.

    In test_exit_status line 73:
            local actual_stderr=$(mktemp -t test_cli_exit_status_stderr_actual.XXXXXX)
                  ^-----------^ SC2155: Declare and assign separately to avoid masking return values.

Reported-by: shellcheck
Change-Id: Iddb1c4e8b0a3bc1c37cc08a37b1103d97b8560e7
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2416
Tested-by: jenkins <jenkins@lttng.org>