Returns a selector for a test case which returns helper test record that
can be used e.g. to render test record steps before the test case is actually executed (or re-executed).
The test record includes the default parameters from the test record (or the defaults from the test run,
if they are not filled in the test record, or the test record does not exist yet). If the parameter is empty,
it will be set to null
Parameters:
iterationIndex - of iteration to be (re)executed; must not be negative, the test record for the iteration may not exist yet