Avoid caching schemas for tests when store_failures is not enabled

[jtcohen6]

In dbt v0.20.0, tests are now configured by default to use a schema named dbt_test__audit, which is useful for the "store failures" feature. On some databases, this is causing dbt to run an extra metadata query when --store-failures is not enabled.

On Snowflake, because of how we list objects to populate the relational cache, such a metadata query often appears in the query history as a failed statement (show terse objects in database.dbt_test__audit), leading to user confusion.

https://github.com/dbt-labs/dbt/blob/43642956a21571e4589d3f7b9a7ad69439c6d4cc/plugins/snowflake/dbt/adapters/snowflake/impl.py#L135-L141

I view this as a low-priority cosmetic issue, with a simple fix: We could change the logic in _get_cache_schemas to check for relational nodes (ones that map to database objects) instead of executable nodes (ones dbt knows how to compile/execute):

https://github.com/dbt-labs/dbt/blob/43642956a21571e4589d3f7b9a7ad69439c6d4cc/core/dbt/adapters/base/impl.py#L304-L316

            for node in manifest.nodes.values()
            if (
                node.is_relational and not node.is_ephemeral_model
            )

The is_relational method already includes logic around whether to consider tests relational:
https://github.com/dbt-labs/dbt/blob/43642956a21571e4589d3f7b9a7ad69439c6d4cc/core/dbt/contracts/graph/parsed.py#L127-L133

And this is the same method we now use inside runnable tasks, to determine which schemas need creating:
https://github.com/dbt-labs/dbt/blob/43642956a21571e4589d3f7b9a7ad69439c6d4cc/core/dbt/task/runnable.py#L460-L462

This would remove Analysis, Operation, Documentation, and RPCCall from the schema-caching list, but frankly it doesn't make much sense that they were there in the first place!