CometTestBase should no longer set the scan implementation to `native_comet`

[andygrove]

What is the problem the feature request solves?

Comet's default scan impl is now auto and we expect that to cause scans to use native_iceberg_compat in most cases now, but our tests run with native_comet explicitly set.

I would like to have the tests use the default of auto instead and update any failing tests to explicitly set native_comet so that we can easily audit which tests require this scan.

Describe the potential solution

No response

Additional context

No response