(maint) update cron whitespace strip acceptance test

Since f5d236702 leading and trailing whitespace was stripped from the
cron command to ensure writing cron entries is idempotent. This made the
cron `should_not_rewrite_with_trailing_whitespace.rb` acceptance test
meaningless. This commit updates the acceptance test to confirm the fix
for #656 and updates the test name to reflect the expected behavior.
Merge pull request #1566 from adrienthebo/maint-cron_acceptance_test_failure

(maint) update cron whitespace strip acceptance test