Use `Realm::Config` for the async open configuration
Use `Realm::Config` for the async open configuration

Using `ServerTestFile` results in the Realm file being deleted when the
configuration goes out of scope, completely negating the point of the
test. This was an oversight introduced while tidying up the test.

realm/realm-core,realm/realm-core,realm/realm-object-store,realm/realm-core,realm/realm-core,realm/realm-object-store,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-object-store