Fix Pandas FutureWarning about integer indexing

[NickFishr]

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

   Fixes SNOW-NNNNNNN

   Github Issue SNOW-1552772: FutureWarning from Pandas about indexing in mock._pandas_util #1967

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
     • If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am adding new credentials
   • I am adding a new dependency
   • If this is a new feature/behavior, I'm adding the Local Testing parity changes.

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

   Pandas prefers integer indexing with the .iloc[] accessor instead of implicitly assuming that an integer key is not a label but an index. It also issues a FutureWarning that in the future also integer keys will be treated as labels and not as integer index, which will most likely cause an error when this behavior is implemented.

   The former code used either used data[x] instead of data.iloc[x] or (maybe unintentionally) data.iloc[x][y] instead of data.iloc[x, y] and caused this warning.

   Not sure if an additional test is really necessary to check for warnings in the future, but I've added it anyway, please feel free to omit the test from the PR if you don't see it as necessary.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[NickFishr]

I have read the CLA Document and I hereby sign the CLA