[System] Rewrite Dns tests so that failures don't cause the process to crash.

[System] Rewrite Dns tests so that failures don't cause the process to crash.

Previously the tests were written so that some failures might occur on the
threadpool in methods that did not catch exceptions. In that case the process
would terminate due to an unhandled exception. So rewrite the tests to catch
exceptions in any code that doesn't run on the main thread, and forward those
exceptions to the main thread.

Also wait for async operations to complete on the main thread, and remove
asserts that are not supposed to be hit.