Add check for max number of allowed characters

[claire-1]

This change was requested in this issue https://github.com/lyda/chkcrontab/issues/22, and I have run into issues with crontab failing because of commands being too long, so having the linter catch this issue will be a big help.

This PR adds a check to catch lines that are longer than 998. It updates the test file as well. According to the crontab man pages, the maximum number of characters in the command field is 998 https://manpages.debian.org/jessie/cron/crontab.5.en.html.

[gregorg]

Please make a PR to my fork: https://github.com/gregorg/chkcrontab