Add missing assertions to run function tests

[Bhoomika910]

Description

This PR addresses Issue #1646 by adding missing assertions to run function tests.

Changes

• Added an assertion to verify that _to_xml() returns a string.
• Added an assertion to verify that trace_to_arff() returns a dictionary.
• Added an assertion to verify that downloaded runs contain parameter settings.
• Added an assertion to verify that runtime is present in run attributes.

Testing

• git diff --check passes.
• Targeted tests were run.
• One test involving the OpenML test server failed because the server returned a document_missing_exception for flow 42, resulting in an XML parsing error. This appears to be a test-server/environment issue rather than a failure caused by the added assertions.

Fixes #1646

[Bhoomika910]

Hi maintainers,

I have implemented the missing assertions described in Issue #1646.

The changes add assertions for XML output, trace ARFF output, downloaded run parameter settings, and runtime attributes.

I also ran the relevant tests. One test-server-dependent test encountered a server-side document_missing_exception, which appears unrelated to these changes.

Thank you for reviewing the PR!