render func.sysdate as SYSDATE()

[sjhewitt]

Please answer these questions before submitting your pull requests. Thanks!

1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes https://github.com/snowflakedb/snowflake-sqlalchemy/issues/289 / SNOW-589556

2. Fill out the following pre-review checklist:

   • [x] I am adding a new automated test(s) to verify correctness of my new code
   • [ ] I am adding new logging messages
   • [ ] I am adding new credentials
   • [ ] I am adding a new dependency

3. Please describe how your code solves the related issue.

   This adds the visit_sysdate_func method to the SnowflakeCompiler to handle func.sysdate() elements